My favorite fiction book is also (what I consider to be) The Room (2003) of literary fiction.

Siegfried is a Adolph Hitler self-insert fanfiction written by globally acclaimed author Harry Mulisch (who is in fact a very talented novelist, let me be clear). It goes something like this: Rudolph "I'm totally not Harry Mulisch" Herter has got it made, hes the best writer ever, he just published the greatest thing ever written, hes got a hot, sexy wife, a smart son, and hes going on tour to talk about the greatest thing ever written, which is a book he made, because he is such a good writer. Though when you're the best, where do you go from there? "I know!" he proclaims on a big tv talk show "I will write a book attempting to process and fully understand the evil of Adolph Hitler! This is something no one has ever done before!" He is later approached by an elderly couple who is like "Yo hey we raised Hitlers secret son back in the day, AMA."

What follows is a bizarre, though interesting character study on Adolph fucking Hitler that plays with his humanity, contrasted with the inhumanity he perpetrated as the leader of the Nazi regime. However, this is interspersed with chapters set in the present time, which is all about the writing process and the musings of a man looking back on his life, career, and art. Those parts are straight up masturbatory and hard to get through. It's much like The Room in the sense that it's a story about its creator, the creators ego, and can be very cringy at times.

I don't know if I recommend it to anyone, and I admit that this is only one way to read it because it is also about the history of Europe in the 20th century and his own personal relationship with it as an artist and a writer, which is what the novel The Assault is about as well. However, if you strip that context away and read it as a piece of entertainment it is an absolute trip. This goes without saying but it is extremely liberal.