1) Digital Photography All-in-One Desk Reference for Dummies
This is a great guide to photography itself, with less emphasis on the geekie side of digital photography. Not for absolute beginners (you should have a basic grasp of the concepts already), but ideal for anyone who wants to learn how to take good pictures with a digital camera.

2) Mastering Digital Photography
This book does a great job of both explaining how digital cameras work (and differ from film), and teaching readers how to take good pictures with digital cameras. I like the breakdown by photo type (action, macro, portraits, and so on).

3) Rick Sammon's Complete Guide to Digital Photography
Using photos as examples to guide readers, this is a great book for people of all learning levels who are switching to digital. It is broken down into a series of lessons.