Infants and Mothers: Differences in Development - T. Berry Brazelton
A very good book that follows the first 12 months of three infants, each on a different developmental curve. It really helps the new mom to realize that month-to-month milestones are relative and that each child has their own schedule.
The Magic Years: Understanding and Handling the Problems of Early Childhood by Selma Fraiberg
Another good read. This book covers more of the cognitive issues met by parents raising small children, how they live in a world where magic is a daily occurrence and every word uttered is taken literally. After reading this book I had a whole new respect for language acquisition and abstract thinking.
Touching: The Human Significance of the Skin by Ashley Montagu
Yet another great read. This book covers the more kinesthetic aspect of parenting, from nursing, massage, skin-on-skin contact etc. There's a reason you don't want to put that baby down!
Miss Manners Guide to Rearing Perfect Children by Judith Martin
Of course, a good read; anything by Judith Martin is a pleasure to pick up. This is the only advice book I kept after the kids were past elementary school. It is informative, precise, funny, easy to read and entirely correct. Want to know how to handle children and "Thank you" notes? Read pages 141-143.