I'll have to split it into categories:
Fiction: War and Peace - Spent a pretty amazing summer reading this. It felt like less of a novel and more of a life experience. I don't know of any fictional work that has its amazing combination of grand, epic scope and small, intimate detail. Combine that with a cast of some of the most psychologically alive characters ever and it's pretty amazing all around. People balk at the length, but it's a surprisingly fast read because of how entertaining it is, and I even slowed down near the end because I didn't WANT it to end!
Drama: Hamlet - I've often said if I'd read this before I saw Evangelion it probably would've had the same effect on me because Hamlet is dealing with a lot of the same shit as the characters in NGE, and by extension what I was dealing with. It's similarly inexhaustible in its substance: the richness of the characters, language, philosophy, etc.
Poetry: Paradise Lost - It's the work that ignited my love of poetry. I still remember reading it the first time thinking "I didn't know language could do this." Since then I've reread it twice and listened to it on audiobook twice, and it's just as amazing every time. Still probably the single richest treasure trove of English language as an aesthetic object in itself.
Non-Fiction: Rationality: From AI to Zombies (https://wiki.lesswrong.com/wiki/Rationa ... to_Zombies
) - Though I read it all online, it's been collected in book-form since. Yudkowsky's fundamental sequence on rationality was like a washing machine for my brain in my early 20s. Much of what he wrote I'd already thought about, but reading him allowed me to clarify it to a great extent. There was also plenty of new stuff to chew on and digest--of those "A Human's Guide to Words" had the biggest impact--and it really helped solidify my overall philosophical perspective.
"As far as we can discern, the sole purpose of human existence is to kindle a light in the darkness of mere being." -- Carl Jung