In no particular order:

One Flew Over the Cuckoo's Nest (much better than the movie) and Sometimes A Great Notion by Ken Kesey

To Kill A Mockingbird by Harper Lee (one more vote!)

The Power of Myth (Bill Moyers interviewing Joseph Campbell)

The Grapes Of Wrath and East Of Eden by John Steinbeck (my favorites, but any Steinbeck is worth reading)

Mila 18 by Leon Uris (again, all his books are good)

The Killer Angels by Michael Shaara

Goodbye Darkness by William Manchester (my favorite, but everything by him is good)

The Crystal Cave, The Hollow Hills, The Last Enchantment, and The Wicked Day by Mary Stewart (a four-book series)

The Prophet and Jesus, Son of Man by Kahlil Gibran

Lonesome Dove by Larry McMurtry (I wouldn't have thought of this, but was reminded by other folks who posted; not a life-changing experience, but a great yarn)

Not So Wild A Dream by Eric Sevareid

Civil War histories by Bruce Catton

Anything by Peter Guralnick

And all the other ones that I'll remember as soon as I post this.