Wesley S started a fine thread on re-readable books. That got me to thinking about the books I use most often and most of them are reference books (so often that I had to buy them). So:

American Heritage Dictionary

Oxford English Dictionary (in 2 volumes and came with a magnifier!)

The Interpreter's Dictionary of the Bible

Strong's Exhaustive Concordance

Ardnt and Gingrich Greek Lexicon

The Nestle's Interlinear Greek-English New Testament

The Septuagint (in Greek)

Biblia Hebraica

The New Jerusalem Bible

The World Almanac

The Harvard Dictionary of Music (My son currently has this)

The Encyclopedia Americana (1953) because I've always had it, that's why!

The Scott Catalog of US Postage

Those I use the most. Rap, if you weigh in here I'd be interested, not necessairly that you "own" them but you do have a wide selection available to you, don't you?