Oooh, one of the better Ask Slashdot's in a while IMHO.
To the OP:
The Elements of Style - Makes you a better writer after reading in one sitting
How to Win Friends and Influence People" - Previously mentioned, but essential. Still, it is mostly "don't be an asshole".
The Selfish Gene - Realize the REAL power and strength of Darwin's arguments. This shows how Natural Selection 'magically' creates order in a chaotic universe.
Flatland - Also previously mentioned but really essential for breaking your brain in a good way.
War and Peace - The first big "historical fiction" I'm aware of and yet rooted in reality with a great setting to boot (the Napoleonic invasions of Russia). It would be like me writing a book on WWII. The last 100 pgs or so are good but mostly a rant though.
Madame Bovary - This book is hysterical when you realize it is just an antiquated moral justification for why woman should not read books (especially romance novels). The story's still entertaining as I recall.
1984 and
Animal Farm - The usual suspects and see the next to see I'm clearly an Orwell fan.
Down and Out in Paris and London - This is where I learned that the more you pay for food at a restaurant, the more hands have touched the food (among many other things). Also, working in a Parisian kitchen in the early 20th century is like working in fast food now.
Crime and Punishment"Crime and Punishment - Get inside the mind of a thrill killer and realize even crazy people think very rationally. This is helpful when you read the latest crazy doing something inconceivable in the news and now know it seemed very logical to them at the time.
The Doors of Perception and really anything by Aldous Huxley including
Brave New World - The Doors of Perception is of course where the band "The Doors" got their name and it, and the collection of associated essays I read at the time, really bring together why people like shiny things -- i.e., they associate them with god or a higher-being intuitively. I love essentially everything Huxley wrote (with Brave New World my least favorite), but his essays are fucking great!
The Moon is a Harsh Mistress" - Great story, sci-fi, and allegory, a la "Star Wars", wrt to the American Revolution.
in contrast though, don't read:
Stranger in a Strange Land - Total shit, beyond the initial premise, which is good, the book is just gratuitous New Age bullshit and will rot your brain. The only value in this book is defining the term "grok" -- I know I'll get flamed for this but it's true...
I've got a few more, but I need to think about them more...