For genres obsessed with ideas and new concepts, science fiction and fantasy produce a surprising number of comprehensible stories with well-defined settings, sharply shaded characters, and plots that coalesce into a perfectly sensible series of events—once you push through a few twists and reveals.

And then there are the eight books on this list.

These books are brilliant, teeming with incredible speculative ideas. They’re also books that no one truly understands—not completely. That doesn’t mean they’re difficult to read or enjoy, it just means you should plan to read them more than once, do a little supplementary research, and possibly spend a few years arguing them over with your friends. Though even then, your chances of getting everything are basically zero.

The Vorhh and The Erstwhile, by Brian Catling

Catling made a stir with The Vorhh, a story that mixes real historical personages with actual myths (including the titular magical forest, thought to exist in Africa), playing with tropes from several genres to create its own singular category, blending fantasy, steampunk, magical realism, and alternate history. The story continues in The Erstwhile, which sees supernatural beings (who might be awakening angels) opposed to humanity heralding a cataclysmic war. Don’t be fooled by this summary—Catling plays so much with language, point of view, and the progression of time in both novels (with a final volume to come), you have little chance of truly comprehending the whole picture he’s painting. What you can do is enjoy the cascade of great ideas and outlandish concepts, the beautiful, often unsettling prose, and the challenge of uncovering the secrets buried on almost every page.

The Book of the New Sun, by Gene Wolfe

Any comprehensive list of “difficult” SFF books will likely include more than one by Gene Wolfe. The Book of the New Sun is one of the best examples of a tightly-plotted, carefully constructed science fiction novel that nevertheless contains so many buried ideas and hinted-at deceptions, it requires a companion book (or the entirety of the internet) in order to even begin to understand it. What makes the story of Severian the apprentice torturer, exiled from his guild and set on a journey across a dying world, so fascinating is you can read it on a fairly superficial level and understand it as a series of events that form a plot. You might come away thinking the story lacks the backstory you’d usually find in an epic like this—but backstory is there. It’s hidden throughout the book, requiring some work to uncover. It weaves together myths and legends while implying the myths and legends are the same story Wolfe is telling, presented in different forms. Far from frustrating, it’s a book you’ll want to read again and again.

Dhalgren, by Samuel R. Delany

Sometimes described as the Finnegans Wake of science fiction, Dhalgren is known for its difficulty and open-endedness. Most people accept the fact that this story of a city, Bellona, cut off from the world due to some unexplained disaster is not meant to be a narrative in the traditional sense. In other words, don’t look for a real plot. Rather, look for symbols, metaphors, and hints at what Delany is trying to accomplish as he tells the story of Kidd, a man who travels to Bellona after suffering memory loss and experiencing frequent bouts of insanity. The whole book can be seen as a metaphor for the creative process of writing a novel—but that doesn’t quite cover everything, either. In the end, Dhalgren is a book to be experienced and appreciated—not “solved.”

The Female Man, by Joanna Russ

Russ wove feminist themes through all of her works, and many people read the title of this, perhaps her most famous, and assume they know what the story is about. They don’t. In fact, even people who have read the book are probably missing something. The premise is fairly straightforward: four women from four alternate realities find themselves crossing over and meeting each other, experiencing each others’ lives. The worlds range from a utopia where all the men died centuries ago to a war-torn dystopia engaged in a literal war between men and women. So far so good—but Russ writes the entire book in a series of first-person perspectives without always playing fair about who is the POV character, and often jumps from one character to another without warning. The result is a difficult story to keep track of, requiring frequent backtracking. You can read this book five times and still miss parts of it—but each time you go back, you’ll find something new—and thrilling.

Report on Probability A, by Brian Aldiss

Aldiss is not exactly known for his easy, accessible fiction, but he reached what is perhaps the pinnacle of difficult SFF with Report on Probability A, a novel whose basic premise was inspired by the Heisenberg Uncertainty Principle—specifically the part about how observing the state of something actually alters that state. Ostensibly the story is in the form of a report detailing the odd happenings in and around a house in suburban England; three men are watching Mr. and Mrs. Mary from different vantage points. The couple knows they’re being watched, and aren’t happy about it. The whole tableau is being further observed by beings in another dimension or continuum, who argue over whether or not all the people involved are actually human. In fact, observers are being observed, and the act of observing alters behaviors and events, and yes, you’re getting a headache just reading this feeble attempt at a description of the book. Whether the end result is a triumph or a beautiful failure is up to you, but no matter how closely, you read you’ll never quite get the whole picture.

A Billion Days of Earth, by Doris Piserchia

If you haven’t heard of Piserchia, it might be because she stopped publishing, apparently voluntarily, in 1983 after the death of her daughter. In 1976, she released A Billion Days of Earth, a book that’s impossible to grasp on a first read. In the approximate year 3 Million AD, humanity has ascended to a new plane of existence in the clouds and are now referred to as gods, while the Earth has been claimed by evolved, intelligent rats that create metal hands for themselves, and refer to themselves as human. An entity known as Sheen—a silver, shapeless thing—emerges and begins consuming the minds of the humans (which are, let’s remind you, actually evolved rats). That high-level summary doesn’t even scratch the surface of what’s going on in this book, and though you can follow the basic plot easily enough, you’ll be left with the gnawing feeling you’re missing something.

Kafka on the Shore, by Haruki Murakami

When Kafka on the Shore was released in Japan, Murakami’s publisher invited fans to submit questions for the author. They received more than 8,000 responses within days. Murakami himself says there are riddles without solutions in the story, and suggests the key to understanding it is to read it several times. It’s not hard to see why—following two narratives in alternating chapters, the book plays with concepts of identity, reality, and the power of music. The end result is you’re never 100 percent certain anything being described has actually happened—Nakata, a man who can communicate with cats, apparently murders another man dressed like Johnnie Walker (yes, the whiskey guy) who is killing and eating cats, but did he really do it? And was the person he killed the father of the other protagonist, Kafka, who awakens in a Shinto shrine covered in blood? All we can do is suggest that yes, you read the book several times. You won’t solve any of its riddles, but you’ll enjoy Murakami’s story each time, so at least there’s that.

What’s the most confusing SFF novel you’ve ever read?