Solve Your Child's Sleep Problems, by Richard Ferber

This book is a lifesaver. Just when you think you're going crazy, rational help is at hand. This book taught me so much about sleep - it explained why my 8 month old wouldn't go to sleep without being breastfed, explained why she kept waking up at night, also explained why sometimes when woken I felt fine, while at other times I was little more than a disoriented zombie!
Not just for babies either, chapter on night terrors (also very good) and other problems (haven't yet had recourse to use them though).
I liked that the book gives a thorough scientific explanation about sleep and sleep patterns before giving you a plan on how to deal with the problem you've got.