John Green was once a little evasive when it comes to the question of whether or not An Imperial Affliction , by Peter Van Houten — the book over which Hazel and Gus fall head-over-heels in The Fault In Our Stars — is 'real.' The answer you're probably looking for is that, no, An Imperial Affliction is not a book that exists in full in our corporeal world; you cannot find it at your local bookstore or on Amazon. Van Houten is, yes, an invention of Green's for the purpose of the themes and story of The Fault In Our Stars — as is the epigraph to TFiOS, an apparent quote from An Imperial Affliction:

“As the tide washed in, the Dutch Tulip Man faced the ocean: ‘Conjoiner rejoinder poisoner concealer revelator. Look at it, Rising up and rising down, taking everything with it.’ ‘What’s that?’ Anna asked. ‘Water,’ the Dutchman said. ‘Well, and time.’ — Peter Van Houten, An Imperial Affliction

In a blog post from two years ago Green responded to a reader who'd nailed down the origin of An Imperial Affliction's title — the Emily Dickinson poem 'There's a certain Slant of light' — to wax (kind of) poetic on 'reality' in relation to the epitaph:

I’m not going on record as to whether the quotation from Peter van Houten’s novel An Imperial Affliction used as the epigraph to The Fault in Our Stars is “real,” because to do so would mean bringing up all kinds of questions about what constitutes “real” when you’re talking about fictions. But there’s no question that the title of An Imperial Affliction is indeed taken from this poem.

One of Green's professed favorite books, The Great Gatsby, also includes a fabricated epitaph. That one read:

“Then wear the gold hat, if that will move her;

If you can bounce high, bounce for her too,

Till she cry, “Lover, gold-hatted, high-bouncing lover,

I must have you!”

— Thomas Parke D’Invilliers

Both serve the purported purpose of an epigraph — to establish a book's themes at the get-go — and both are credited to fictional authors.

Green, it could be argued, took his to the next level: Peter Van Houten is a crucial character in The Fault In Our Stars, and 'his' words don't just kick off the book then disappear — both his thematic and his physical presence is felt throughout the entire story. He plays a huge role in one of the book's themes, and one that often gets overshadowed by its larger dealings with death and romantic love: The love one has with a story — or with the author who created that story.

In that way Van Houten serves both as a proxy and a foil for Green's persona. As he wrote in a FAQ for TFiOS when asked if Van Houten is at all based on him:

Sure, yeah. I mean: 1. Happily, I am not an alcoholic. 2. Sadly, I do not have an assistant, let alone a beautiful Dutch assistant. 3. I am not particularly reclusive. 4. I hope that I do not use pomposity and pretension to shield myself from trauma. 5. Most of the bad things that have happened to Peter Van Houten have not happened to me. 6. I am somewhat younger than he is. However: 1. I also like Swedish hip hop. 2. I share PvH’s belief that books belong to their readers, and that authors are not qualified to comment on what happens after their books have ended. Like PvH, I am often asked about what happens in my books after they ended, and like him, I have no answer. 3. Like PvH, I am I guess somewhat depressive and very introverted and therefore can get overwhelmed by readers’ expectations of both me and my work. 4. I know what it’s like to feel that I’ll never be able to write anything worth publishing ever again. 5. I think sometimes I probably do intellectualize emotionally painful experiences so that I don’t have to confront/process them emotionally. 6. I also understand set theory better than Hazel Grace Lancaster does. :)

An Imperial Affliction is a book-within-a-book; you can't go out and buy it, though you can gaze upon the beautiful book covers fans have designed for it. Similarly, Peter Van Houten is a sort of author-within-an-author; he's not a physically existing human, and he's not John Green, even though he came from John Green. But both Van Houten and An Imperial Affliction came from a few other things as well, as Green explores here:

I get asked this question all the time, often by journalists. (I won’t name any names, but a pretty well-known journalist once asked me how Peter Van Houten felt about my depiction of him.)An Imperial Affliction is not a real book, and Peter Van Houten is not a real person. However, An Imperial Affliction is in some ways based on two books I love. The first is David Foster Wallace’s Infinite Jest. Most of the references Hazel and Augustus make to AIA are related in some way to something from Infinite Jest, and I wanted readers of IJ to be able to make those comparisons. But Infinite Jest is not about cancer. Peter De Vries’ amazing and beautiful and hilarious novel The Blood of the Lamb IS about cancer, and most of the broad observations that Hazel makes about An Imperial Affliction — how it is a book about cancer without it being a cancer book, how is is funny and respectful and reflects the reality of experience in a way she has rarely encountered—come from my own experience reading The Blood of the Lamb. I can’t make An Imperial Affliction real. It’s not the kind book I could write well, and on some level, the thing that we imagine will always be better than any real approximation of it that might come to exist. But if you wish to read An Imperial Affliction, I’d encourage you to read Infinite Jest and The Blood of the Lamb and then try to blend the feeling of those two books.

If you desire to know more about An Imperial Affliction and its story and themes, you can read up on that here.