Maybe "The Stranger" by Albert Camus. I don't know if this is my favorite book. I just listened to its audio book a lot.

Anything in the Dialog series by Neale Donald Walsch.

Probably Catch 22 or Catcher in the Rye because I love absurdity. And also "Lies and the Lying Liars who tell them" by Al Franken because I never laughed so hard at anything.

Sidbey Sheldon,Alistair Maclean

"At swim-two-birds" by Flann O'Brien. Or did you mean "adult" like in "erotic" books?

The Count of Monte Cristo has been my favorite since the teenage years - first because I was a bitter kid who'd been wronged and got off on the revenge tale, then as I got older I came to appreciate how toward the end, the count saw the damage his schemes had wrought in all these peoples lives, and wondered whether it was worth it in the end

Snoopp Dog's cookbook