Solve Your Child's Sleep Problems: New, Revised, and Expanded Edition by Richard Ferber, M.D.: Dr. Ferber's classic guide to sleep training. The Ferber method is one of the few methods to have been studied in clinical trials. It has been proven to reduce the severity of postpartum depression in a group of depressed mothers in Australia. That's not to say the method is good, bad, or the right one for you. I'm just pointing out that we actually have some clinical data about this method. Note: Wendy Hall, a UBC researcher, has pointed out, quite rightly, that we need a lot more research into the relative merits of various sleep training methods. She says it's no wonder we have so much conflicting evidence -- and that parents are so confused. She says we need to treat sleep research as a priority, just as we treat nutritional research.
Sleeping Through the Night, Revised Edition: How Infants, Toddlers, and Their Parents Can Get a Good Night's Sleep by Jodi Mindell, PhD: Jodi Mindell does a fabulous job of explaining infant sleep patterns and all of her sleep recommendations/tips are solidly backed by research. Her tone is warm and reassuring and her credentials are impeccable. (She is one of America's top sleep experts.) Even if you find the cry-it-out method a bit too tough, you can still pick up plenty of valuable information from Mindell's book.
Healthy Sleep Habits, Happy Child by Marc S. Weissbluth, M.D.: Marc Weissbluth's book has a lot of useful information on how sleep cycles evolve in babies and young children and what parents can do to promote healthy sleep habits. You don't have to agree with everything he says in this book or decide to use his sleep training methods to glean very valuable information from this book that you can then apply to your child/family's situation. Consider this a really solid "sleep science" title.