If you enjoy mindfk books, I'd recommend House of Leaves by Mark Z. Danielewski. It is a very strange piece of fiction, I think it's sort of horror? But more mental than anything, and the book itself is gorgeous, with interesting typesetting and strange, spiraling passages, blank pages.... a story within a story within reality. It's not for everybody, but I've never read anything quite like it. It's the only book I've read that actually kept me up at night, a bit scared and uncomfortable, heh. But it's not for everyone.
If you like sci-fi, anything by Philip K. Dick is great, especially Do Android Dream of Electrtic Sheep, which was turned into the movie Blade Runner. The novel itself is brilliant and also very different from the (excellent) movie.
Neverwhere and American Gods by Neil Gaiman are great, sort of dark fantasy. I think I read Neverwhere in about a day, it's great.
Kurt Vonnegut is great if you haven't checked him out, Slaughterhouse Five and Cat's Cradle especially.
If you've never read the graphic novel The Watchmen, you should do so. It's the only graphic novel on the Time 100 Great American Novels list, and for good reason. It's smart and powerful and still relevant today, despite being set in the 80's cold war era.
I also second recommendations for Hitchhiker's Guide to the Galaxy, which is my all-time favorite series, so funny, so smart, so good. I reread them every couple of years. Also the A Song of Ice and Fire (Game of Thrones) books are great, but as others have said, they're long and unfinished sooooo.
This is my Goodreads profile if you want to see what else I rated highly.