As Donald Trump was hitting his stride at his White House press conference this week, doing what he does best – denigrating journalists, spreading falsehoods about his electoral victory, offering science lessons on the uses of uranium – text messages started pinging on the cellphones of news anchors sitting in the front row.

“He should do this with a therapist, not on live TV,” one text reportedly said. It came from an interesting source: a Republican US senator.

It is one month into the Trump administration and all of Washington, most of the United States, and sizable chunks of the globe are transfixed by the new White House incumbent. While his supporters remain ecstatic about his entirely unorthodox approach to the most powerful job on Earth, millions of others – among them, it seems, Republican senators – are scratching their own heads and wondering how to understand what is going on inside his.

The good news is that help is at hand, from an unexpected quarter. Roger Stone, a longtime confidant and adviser to Trump whose dealings with the New York real estate mogul stretch back almost 40 years, has written a new book that gives fascinating insights into the mindset of the man now nominally leading the free world.

The Making of the President is billed as an in-depth examination of how Trump, whom Stone calls the “greatest salesman in US history”, came to defeat Hillary Clinton in 2016. Stone played a hand in that – he was there at the birth of Trump’s campaign as a senior adviser but was forced out, having clashed with other key members of the team.

Among the revelations in the book is the fact that Trump quietly trademarked his catchphrase “Make America great again” within days of Mitt Romney being defeated by Barack Obama in the 2012 race. But its most enlightening quality is what it tells us about the contorted, volatile psyche of its author – and by extension, of his friend Donald Trump.

It’s fair to connect the mindsets of the duo because Trump and Stone have for so long had so much in common: they both have an obsession with presidential politics that goes back to Richard Nixon, for whom Stone worked in the White House; they both have an intense fondness for conspiracy theories, most notably the “birther” lie that Barack Obama was born in Africa, of which they were leading proponents; and they both, as we shall see, have a very loose attachment to the truth.

In an interview with the Guardian around the publication of his book, Stone gave a peek into his thinking that in turn goes some way to explain the baffling worldview of the 45th president of the United States. The lobbyist revealed the dystopian universe he inhabits in which people can choose the truth to which they subscribe.

Stone began by discussing how Trump courted a plethora of new rightwing websites such as Breitbart and Infowars during the election campaign as a way of bypassing mainstream media and reaching out to a previously untapped audience of disgruntled white working-class voters. “This is the election in which the tipping point was reached,” he said. “The pendulum swung away from the mainstream media, which lost their hammer-lock monopoly on the dissemination of political information.”

He went on: “Now you have a whole constituency of alternative media out there for Trump to appeal to. He very skillfully worked with that alternative media.”

It’s hard to disagree with that analysis. But the conversation got weirder when the Guardian asked Stone about the potential downside of the rise of “alternative” news.

Isn’t there a danger that information becomes entirely partisan – with half the country drawing its knowledge from rightwing websites and the other half from more liberal outlets? In that case, aren’t we all left at sea, not knowing what is true and what is false?

“I disagree,” he said. “At least people now have a choice. They can believe one outlet or the other outlet. Under the old system, they are only fed one truth, there was no alternative presented. When you have NBC, ABC and CBS moving in lockstep, then if they said something didn’t happen, it didn’t happen.”

So there can be a choice over the truth? Yes, he said.

Take JFK. “There used to be only one version – the government’s version – about the assassination of President Kennedy: Lee Harvey Oswald killed him acting alone. There was no argument over that until the internet got into full swing. Today, a majority of Americans do not believe Oswald killed Kennedy acting alone – it’s amazing, the people have come to what I believe to be the correct conclusion. Without an alternative media today, you wouldn’t even have this debate.”

You can choose your truth. Those five words help understand some of the more bamboozling aspects of Trump’s press conference this week.

You can choose the truth as told by the mainstream media that Trump won the 2016 election by a slimmer electoral college margin than Obama or Bill Clinton, which happens to be true, or you can choose the truth spoken by Trump on Thursday that he had enjoyed “the biggest electoral college win since Ronald Reagan”, which happens to be false. Take your pick.

Stone’s own complicated relationship with the truth stretches back decades, running parallel to his friendship with Trump. In the 1970s he was part of a group of advisers around Nixon, his lifetime hero whose face he has tattooed across his back. The Nixon acolytes called themselves “ratfuckers” in a self-congratulatory reference to their proficiency at the darker arts of politics.

Stone said he had never applied that label to himself, implying that he dislikes the expression. “Ted Cruz called me that, but that motherfucker should just look in the mirror. If you want to see sleaze, just look in the mirror.”

He also bridles slightly at the mention of the other phrase that is frequently applied to him – dirty trickster. “One man’s dirty trick is another man’s civic participation,” he quips, before launching into a defense of his notion of politics as a tough, violent affair.

“Politics is not beanbag. This is a contact sport, always has been, always will be. It was alleged that Martin Van Buren dressed up in women’s clothes, that Abraham Lincoln fathered mulatto children – this is part and parcel of American politics.”

But it would be hard not to conclude that “dirty trickster” is a fair label to pin on a man who firmed up his reputation for hardball tactics working on Ronald Reagan’s presidential campaigns, had a hand in the prostitute-laden downfall of the former New York governor Eliot Spitzer and consulted for a number of controversial foreign clients, including the president of the Philippines, Ferdinand Marcos. Toward the end of the 2016 presidential race, he launched an initiative to protect against voter fraud that was widely denounced as a form of voter intimidation. In his book, he also boasts of having conceived the idea of campaign T-shirts to be sold at Trump rallies modeled on the legendary Obama “Hope” design, with the image of Obama replaced by Bill Clinton and the word “Hope” exchanged for “Rape”.

When you combine Stone’s reputation for dirty tricks with his belief that we have a “choice” over truth, it gets rather slippery. How is one to know whether he adheres to a position because it is his “alternative” truth, or because it suits his purposes, or both?

When he issues the Guardian with a resounding denial that he had anything to do with links with the Russian government at a time when the Kremlin was busy meddling in the election race in favor of Trump, is that true, his choice of truth, or a convenient spin? Or all three? How are we meant to distinguish between them, having been drawn into his dystopian world of competing realities?

“I can speak for myself,” Stone said, having been named by the New York Times this week as one of four Trump associates who have been investigated by the FBI over possible contact during the election with Russian officials. “There was no collusion; I have had no connection with the Russians. If the government has evidence that I was colluding with the Russians in Donald Trump’s campaign, they should indict me immediately.”

The Making of the President is packed with vivid and often offensive conspiracy theories. Not content with having played a part in the defeat of Hillary Clinton, Stone has to kick her when she’s down. She is suffering from Parkinson’s disease and dementia, he informs the reader. Bill Clinton fathered a son by an African American prostitute. The protests at Trump events both before and after the election were organized and funded by that favorite hate figure of the right, the financier George Soros. The former head of the CIA, whom he blames for putting out a false story about Russian hacking in the election, is a Muslim and a Saudi spy.

Stone draws liberally from the far-right outlets that he praises as having finally provided the American people with a “choice” over the truth. He cites without qualification the notorious conspiracy theorist Jerome Corsi and rightwing provocateur Daniel Horowitz.

He also leans heavily on Infowars, the website and show on which Trump has controversially appeared. Infowars’ host, Alex Jones, whom Trump has lauded for his “amazing reputation”, is responsible for spreading some of the most loathsome conspiracy theories invented, such as the line that 9/11 was an inside job or the story that the Sandy Hook massacre, in which 20 six and seven-year-old children were brutally gunned down, was a hoax.

“Alex Jones is reaching four times the number of people reached by Fox,” Stone said. “It’s a sizable outlet. I don’t think you have to agree with everything said by Alex Jones to be interviewed by him.”

As for the mainstream media, he is magnanimous enough to say that it is not “monolithically dishonest – there are honest reporters just trying to do their jobs. But there are some in the mainstream media who are hopelessly biased and some pushing fake news.”

He goes on, warming to his theme: “Breitbart News is doing better investigative reporting than the Washington Post. Just because a news outlet has a blue-chip name does not mean they are still doing exemplary journalism.”

The Washington Post, of course, is the paper that brought down his hero by exposing Watergate. Now it is at the forefront of exposing the Russian contacts, conflicts of interest and taxpayer expense of Trump’s White House.

But there’s a difference between the Post of the 1970s and the Post of today, at least in the strange worldview of Roger Stone and Donald Trump. There was no resisting the truth of Watergate. But this time around, truth is a matter of choice.