(CNN) Francisco Cantú's first clue that he needed to quit his job as an agent with the Border Patrol came in the form of dreams. He would have nightmares "encountering dead bodies in the desert or that people I had arrested were coming back to me." He ignored those dreams at first, but they wouldn't go away. "I had them all the time," he says. Soon he was dreaming that his teeth were falling out.

"And a couple of years after I started," Cantú remembers, "I went in for a dental appointment and my dentist said that I was grinding my teeth in my sleep -- so much that I had worn through several layers of enamel in my molars. So that was the first time that I saw a real-world connection with what I had ignored as just dreams coming into my real life."

The Mexican-American grandson of border crossers whose mother spent a career with the National Park Service, Cantú narrates unsparingly in his recent book, " The Line Becomes a River: Dispatches From the Border ," the years he spent policing the US-Mexico border and afterward, when he doggedly tried to prevent a close (and undocumented) friend, Jose, from being sent back across it. Jose, who had lived in the United States for many years, returns to Mexico to say goodbye to a dying relative and gets apprehended trying to re-enter the country.

Cantú, now 32, joined the Border Patrol out of college to try to understand the Southwest landscape he'd grown up with and that seemed so bound together with his personal history and academic interests. As he puts it, joining the Border Patrol "seemed like ... the only way to see the reality of the border and to be out in the desert, day in and day out."

What Cantú shares in his book is the unexpected truth that after years spent walking, tracking and investigating out in the desert along the border, it was only after he quit that he, in his own words, "approached having an understanding of the extent to which the border can really rip through people's lives." After leaving, he pursued a Fulbright fellowship and became a writer.

At one point in "The Line Becomes a River," Cantú's mother, an avid outdoorswoman, reflects on her time spent with the Park Service and says, "The government took my passion and bent it to its own purpose. I don't want that for you."

Cantú admits he struggles still with the question of whether the border is inherently a place where individuals, no matter which direction they're crossing, risk being overtaken by systems of power -- wall or not, reform or not. Part of the work in writing this book, he says, "was thinking about all the ways in which I was lending these parts of myself and my identity to help enforce these flawed policies that when I look at them now, feel violent and inhumane."

CNN Opinion spoke with Cantú about his experience writing a book about the border, one that in an era of "immigrant" being used as a dirty word, seeks to heed historian Timothy Snyder's call (which Cantú quotes in his book) to "turn the numbers back into people. "

This interview has been edited and condensed for flow:

CNN: As someone with a unique experience of the border, can you describe what the few last years have been like for you, as the border increasingly occupies a very particular place in our national political discourse?

Francisco Cantú: When I listen to today's conversations, to be honest, I don't feel that conversations about immigration or border security have changed or evolved all that much. When I was in college in 2006, we were having another iteration of the same sorts of fight on immigration reform. It was constantly up for debate in the House and the Senate. Minutemen militias were forming. I don't think we've come very far since then.

I also just think that our conversation doesn't acknowledge the complexity of this place, the border. We talk about the border as if it's some frontier at the edge of our country that is so disposable that we can just build a wall across it.

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We don't talk about the human cost of that policy. It's because we have walls and extreme enforcement in urban areas that people are being pushed out to cross in the desert. And our current policy weaponizes the landscape in that way.

The reason that "build the wall" has become this mantra is because it sounds so easy, and there's comfort in that, I think, for a lot of people -- especially people who live far away from (the Southwest) to think that we could just "build a wall." It just sounds easy and it sounds like a fix, so it's sort of "out of sight, out of mind." But these are issues we're going to be grappling with for a long time, and we have to reject anything that sounds like an easy fix.

CNN: Where did your fascination with the border come from?

Cantú: I grew up in the Southwest. My mom was a park ranger, so I was always very close to the outdoors. Then I went away for college to study in D.C. And I didn't really become obsessed with the border until I left the Southwest, in a weird way. In the course of my studies, I focused on immigration and border issues, and when I was getting ready to graduate, I knew that I wanted to come back to the Southwest. I felt like there were all these things that I was missing, and since I had a sense of the landscape, I felt really disconnected from what I had been learning (about the border) in books and articles and studies. So I really wanted to know what the border felt like. And I wanted to be out on the hottest days and the coldest nights. I was looking for answers.

CNN: That's a powerful reason to join the Border Patrol. When did you realize you wanted to move on from it, and what was it that precipitated that change?

Cantú: I did the job for about 3½ years, and part of the Border Patrol, like any military or law enforcement agency, is conditioning you to accept the realities of that job as something normal. To the extent that you can do it day in and day out. And so I never would have told you if you asked me during that time that I was stressed out, or that I had misgivings about the job. I put a lot of that stuff away in order to do the work. The first indication the job might not be for me came in the form of nightmares that I would have.

CNN: There are a number of striking encounters you describe in the book with people seeking to cross and residents who live at the border. Of those, what are some of the ones that most memorably stay with you?

Cantú: The encounter that has made the biggest impression on me and that I still carry with me more than any other was finding this body out in the desert. He was a middle-aged man who crossed the border with two kids, 19-year-old kids -- one was his nephew and one was his nephew's friend from the village where they came from. You hear about the fact that people die in the desert. You hear about rescue operations and stuff like that. But for me, seeing that for myself is something that I'll always carry with me. I still remember that man's face. I remember the ants crawling across his body -- these very visceral details that make it real.

It's so abstract, I think, when we read about it (the fates of border crossers or immigration issues more generally) in the news. And I really think that I came away from that experience really thinking that this is a humanitarian crisis that we don't acknowledge, that we don't talk about in our discourse. Hundreds of people die along the Southwestern border every year, and the ones we count are only the ones we know about. So I think the fact that we don't acknowledge those people, we don't mourn their deaths, we don't claim them, they remain unidentified -- that part is unacceptable. I think that should be the first thing we address with any type of immigration reform.

CNN: Do you ever talk about issues like the wall, or DACA, with any of the people you met while serving in the Border Patrol?

Cantú: In the station where I worked, we had several miles of 20-foot-tall fencing, and when I would sit around with my co-workers and talk about the wall in our area, we just talked about how silly it was, because smugglers had figured out ways to literally pry open these steel panels and put car jacks underneath them and jack them up high enough that a car could drive underneath. Or they would bring welders and weld little holes or doors in the wall that people could crawl through.

Once there was a breach in the border wall, they would have to assign an agent to sit there at the hole in the wall until a fence repair crew could come down and fix it. And that happened once a week. So sometimes there would be six days where an agent would just be assigned 24 hours a day to sit at the fence. And the fence crew is getting paid by the hour to fix these holes. That's all taxpayer money. I think it seemed insane to a lot of the guys that I've talked about it with. And it seems insane to a lot of people who live in the area, and who have a sense for how rugged and vast the terrain is.

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From what I've seen, I really think that no matter what obstacles we put at the border, it's going to be subverted. People are going to find a way up, over, under or around it. And I've also seen, and we have all the evidence before us, to say that no matter how arduous and hellacious we make this journey across our border, people are going to endure it to get to the other side -- especially when they are separated from family.

And so for me the question becomes: Do we spend our time and money on a wall? Do we ask our policymakers to just keep conceiving of ways to make our border more hellacious? Or do we start thinking about ways to reform our system or alternative ways of providing aid to these countries that are sending migrants and assisting economic development in those countries?

CNN: As you reflect on the experience now and having written the book, do you still think of yourself as "former law enforcement" as part of your identity?

Cantú: I only did the job for 3½ years. For a lot of people, they work in law enforcement their whole lives, and it really comes to define them in a much bigger way. For me, I had a different experience. I joined the Border Patrol and I really looked at it like an extension of my education. I thought that I would go on to become an immigration lawyer or a policymaker. When I joined, I didn't think of it as something I would do for the rest of my life.

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But now, and I think there's always a part of me that has to recognize myself as former law enforcement, the Border Patrol is a part of me now, that work is a part of me now, that I'll never really be able to do away with or move past. It's definitely changed me. I think for a lot of law enforcement, you fixate on these proud moments. The car chase, or a drug bust, or apprehending a criminal. And you pat yourself on the back because you kept that person out of the country. But for me, all that has faded away, and what sticks with me are these quiet, human, interactions that I had with these people that I was apprehending.

CNN: The final movement of the book shifts from your professional to a more personal experience with border crossing. How did becoming part of your friend's life in this particular way -- a way that only you, of all the people in his life, could be involved with him -- change you? Has it changed how you see immigration policy?

Cantú: The reason that the book ends there is because Jose's story really transformed me and really brought home all of these things that I was grappling with in a much deeper, more devastating, personal way. I had arrested countless people and processed them for deportation, but I had never looked behind that curtain and seen what happens after that person is sent away to be processed.

We hear over and over again about the challenges that face the millions of undocumented migrants who live in our country, but like I said earlier, nothing can be as impactful as seeing that firsthand, as connecting with an individual story.

I think that's something that we need right now. We need to be hearing those stories. We need to be hearing from the people who are suffering through these realities firsthand. And I think that as writers and artists and journalists, it's our job to amplify their voices above the others who are speaking so loudly. Because it's otherwise impossible for us to understand the fear that these people are living with. I've talked with people who are literally afraid to leave their homes in this political moment. It's not just rhetoric.

***

As a reader, my first reaction to "A Line Becomes a River" was: What does a bookseller have to do with Donald Trump's border wall?

The short answer is that following the ratification of the 1848 Treaty of Guadalupe Hidalgo, which largely defined the modern-day demarcation between the United States and Mexico, each nation appointed commissioners and surveyors to mark the new boundary that Trump now hopes to make a 21st-century fortress.

And, as Cantú points out, the US contribution to this project was initially "carried out under the questionable supervision of John Russell Bartlett , a well-connected and adventure-hungry bookseller living in New York."

This is far from the only incongruous fact in Cantú's book about his struggle to understand the geographical swath that has cut such a dark, broad fissure into our politics. He cites a cultural sociologist's findings that journalists' most commonly used metaphors in writing about migrant deaths have cumulative, dehumanizing effects. He reports that during the same years when Ciudad Juarez was the " murder capital of the world ," El Paso -- the Texas city just across the border, where Cantú was stationed as a border agent at the time -- was named the safest city in the United States.

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Cantú doesn't present this level of precision detail as a collector of trivia; rather, his infusion of historical information, reportage and academic analysis into what is primarily a memoir of his time spent as a border agent manifests the obsession that prompted him to join the Border Patrol in the first place.

There's a scene in the book when Cantú describes swimming in the Rio Grande and losing track, for a moment, of which side of the border his body is on. By coming to better understand Cantú's fixation with the border, readers of his book are brought into that suspension, prompted -- if not outright required -- to experience what it's like to exist in-between, knowing no amount of politics or prayer can give a hard question easy answers.