I honestly can't say enough good things about this book. Every time I hit a rough spot with our second baby, I found the explanation here, and wished I'd had it sooner.
The Wonder Weeks breaks down the baby's first year or so according to the developmental leaps that occur. Understanding what was going on inside her brain and body helped me to be more sympathetic when my daughter was whining and clingy, or waking up through the night, or just seemed to turn into a different child overnight.
Offering suggestions on how to work through each developmental leap and turn it into an opportunity for learning and for growth, the authors also help to foster a better relationship between parent and child. It is much easier to make it through a particularly difficult week when you realize that your child has suddenly become aware of things like "mommy can leave and I can't stop her" or "I am a distinct person and not just a part of my mommy". Learning new things like motor skills, size & shape relationships, etc. changes the baby's perception of the world and leads to a time of confusion - and it takes a little while for everything to settle again. Patience and encouragement from a loving caregiver will help the baby to get through each stage more smoothly and with the best results for everyone.
I highly recommend this book to all new parents, and as a gift idea for anyone going to a baby shower. This is one of the few books that I literally consider to have been life-changing.