Well I wouldn't be too concerned about choosing books strictly on the basis of their ability to increase my vocabulary or improve my writing style. Rather, I would focus on books that are a joy to read and allow the natural process of learning that follows delight.
Here are some of my favorite books, in no particular order:
Catch-22 by Jospeh Heller.
The Name of the Rose by Umberto Eco. A bit weighty in places (I recommend skimming as necessary) but you can't beat a good murder mystery in a medievel monastery that has a labyrinthine library.
Number the Stars by Lois Lowry. Ostensibly a children's book but worthy of an adult's attention as well. It's set in Nazi-occupied Denmark -- very exciting and hard to put down. And on that subject...
The Moon Is Down by John Steinbeck. Also about a Nazi-occupied country, this time Norway. A little darker than Lowry's book, this novella can be read in one or two sittings.
Into Thin Air: A Personal Account of the Mount Everest Disaster by Jon Krakauer. A creative writing teacher in college once told me that good writing is like a window: you notice the view through the window rather than the pane of glass itself. This is a sort of Hemingway-influenced notion of good writing, and Krakauer's style is in this vein. His book Into the Wild is also impossible to put down. Speaking of Hemingway I just read for the first time...
For Whom the Bell Tolls and loved it.
Another novel set in war, All Quiet on the Western Front, by Erich Maria Remarque is another classic that you'll actually enjoy reading despite Mark Twain's comment on "classics." ("A classic is a book everyone wants to have read but nobody wants to read.")
The poetry of John Donne, such as "A Valediction: Forbidding Mourning", which can be found in any number of books is moving, clever, beautiful, inspiring, you name it...
William Faulker can be brilliant but also a bit of a chore at times. As I Lay Dying is probably a better starting point than The Sound and the Fury, although I survived having the latter thrust upon me in tenth grade and eventually learned to love it.
The Adventures of Huckleberry Finn is hilarious and and sad and all-around wonderful, even if it kind of loses steam towards the end.
Just about any of the short stories by Edgar Allen Poe are great. I read all of those, and all the Arthur Conan Doyle Sherlock Holmes stories I could get my hands on in high school.
A Prayer for Owen Meany by John Irving was so good that I started reading it on a Friday night and finished it in time to return the book to its owner Saturday morning at breakfast in the college cafeteria.
The Lords of Discipline by Pat Conroy is a great read too, and not nearly as corny as The Prince of Tides, although this too I read in about two or three sittings. (One must have his guilty pleasures.)
To Kill a Mockingbird by Harper Lee is one of my favorite books, and the movie is excellent too.
The Catcher in the Rye by J. D. Salinger stays popular for a good reason.
Many of Kurt Vonnegut's books also remain popular. I like Slaughterhouse Five and Cat's Cradle best but you can hardly go wrong with one of his books.
Reading drama alone is a bit like singing harmony alone, but it can be a pleasure to read a play and then watch it on VHS or DVD, or better yet see live if that is an option. For example, there's Cat on a Hot Tin Roof or A Streetcar Named Desire by Tennessee Williams.
Edward Gorey's macabre quasi-children's books are a great pleasure to read both for their wordplay and engrossing pen-and-ink illustrtations.
