A Path with Heart by Jack Kornfield. While I haven't actually finished this book, I think this book was my first introduction to teachings in Buddhism. This book has truly shaped my life, as I continue to have an active interest in Buddhism, mindfulness, loving-kindness, and many other related aspects. If I were approaching the subject afresh, I would probably start with When Things Fall Apart by Pema Chodron, which is rather mistitled, but a fantastic down-to-earth read about some of the most important aspects of Buddhism for personal progression.
-- Oliver Charles