This book has been much more valuable to me than any other on the topic, specifically because it is not "preachy". Unlike "Babywise" and other breastfeeding/infant sleep books, it does not constantly prescribe the "best" way to breastfeed your baby. "The Nursing Mother's Companion" is outlined by the baby's ages and developmental stages, and information about your baby's likely nursing habits within that developmental stage follows. It lets you know what to expect, some potential pitfalls (like plugged ducts, refusal to nurse or overabundant milk) and how to handle them. After reading it, I felt both like I had a handle on what nursing was going to be like and that I would be able to nurse my baby successfully--and I have, largely due to this book.
I have to admit I didn't read the entire book, because I was unable to breastfeed. For those new moms who are going to breastfeed it seems to have everything you might need to know.