Psalms-- Lots of very eloquent passages-- you'll find something that speaks to your mood no matter what mood you're in. Also great illustrations of how to praise God.
Hebrews-- If there was one book in the Bible I'd give to someone wondering what Christianity is all about (assuming they at least knew who Jesus was), it'd be this one. (I know, "what about the Gospels?", but a lot of those passages are relatively "commentary-free" and more simply a historical telling, and some can be confusing without outside help.) Hebrews stands pretty well on its own.
Genesis-- The beginning and the end of the book in particular. There's also definitely some things "missing" (whether intentional or simply lost over time) in the first several chapters (what about Satan's rebellion? The Nephilim come out of nowhere, etc.), which I love speculating about.
Honorable Mentions: John, Romans, Matthew
Also-- and this certainly isn't a swing at the guy-- but whenever I hear "Habbukuk" I think of someone choking.