These are my top 15 books. Yes, I'm aware of how many times Phillip K. Dick and Neil Gaiman come up, but they are my favourite authors. I go insane if I don't consume at least 2 books a month, so I had a longer list but these are the ones I would narrow down.
1. American Gods by Neil Gaiman. This thing is my bible. If you are even remotely interested in mythology and anthropology regarding religion, pick this book up post haste.
2. Do Androids Dream of Electric Sheep? by Phillip K. Dick. The best sci-fi novel EVARR.
3. Naked Lunch by William S. Burroughs. I recommend this one with a bit of trepidation. If you are into Beat poetry and aren't easily offended, snatch this one up. If you are easily offended, don't touch this book with an 80,000,000 foot pole.
4. Of Mice & Men by Jonathan Steinbeck. You've probably already read this book, but this is one of those books that gets put in the "Classic" category that I actually agree belongs there.
5. Anansi Boys by Neil Gaiman.
6. A Scanner Darkly by Phillip K. Dick
7. The Stand by Stephen King
8. The Hitchhikers Guide to The Galaxy (And its sequels) by Douglas Adams
9. Fear & Loathing in Las Vegas: A savage journey into the heart of the American Dream by Hunter S. Thompson.
10. The Man in the High Castle by Phillip K. Dick.
11. Logan's Run by William F. Nolan and George Clayton Johnson. This is an underrated classic, IMO. When I mention it, people think of the campy movie, which is a fun ride but nothing like the book. The book has a great mix of fantasy and sci-fi, with some clever references to various bed time stories that you won't notice on the surface but fit surprisingly well.
12. A study in Emerald by Neil Gaiman. To be fair, this is a short story (About 27 pages) but its ingenious and a very surreal mind fuck. You can find it public domain on the net, I think Harper Collins has a post of it for free. If not its collected in "Fragile Things," a collection of his short stories and poems.
13. Masters of Doom by David Kushner. Good if you like ID Software, its well written and is funny whilst being informative about ID softwares history and whatnot.
14. Shogun by James Clavell. This would be higher on the list, but its a bit windy and there are passages that are completely irrelevant and uncomfortable, like the 40 page chapter about the old man with strange sexual urges and who gets off watching foreigners be boiled alive, but when it stays on focus of the main plot its a great mish-mash of culture and history and is especially a good read if you are interested in Japanese history (The proper kind, anyway. Not that Anime stuff, I find it distressing so many people think they can properly extract history and culture from Anime :|)
15. The Lovely Bones by Alice Sebold. Like Shogun, I want to put this up higher because it is a beautifully written and very haunting story, but the only reason I put it lower on this list is because I've never been able to deal with stories that deal heavily with rape and it was a bit depressing and upsetting. Regardless, it is beautifully written and sticks with you, and has a unique method of story telling.