These are my three most useful books, purchased during theological college and used heavily ever since, don't leave home without them. Wallace's Greek Grammar is like a portable Greek lesson, the Worship Sourcebook provides lots of biblically based liturgy and The Cross and Salvation is a compendium of theological bread and butter. I've also found having a broad range of commentaries more useful then I imagined, I thought they'd sit gathering dust. Interestingly having spare copies of Is Christianity Good for the World, The Prodigal God and The Journey has been very useful evangelistically.