I really enjoyed Einstein Never Used Flashcards, and What's Going On In There. I'm currently reading Pink Brain, Blue Brain, which I recommend with the caveat that I'm not even halfway through yet

The very best childhood development book I've ever read is no longer in print, but I found it at my library. It's called Learning Early and it's by Dorothy Einon. It's the only book I've seen that offers specifics about both physical and cognitive development. I don't agree with everything in the book, but I found it a really great resource.
Whatever you do, DO NOT pay any attention to any "what your baby is doing this week!" emails or lists in books. There are some milestones that are a bit worrying if not met, but 99% of the things on this lists happen during a really wide range and you will drive yourself insane reading them.