May I suggest:
http://www.amazon.com/The-Philosophi...1963532&sr=1-2
It is not an AP book per se, but I think it will support an AP approach by describing how children experience the world at different ages/stages. I plan to start reading it myself soon. Maybe I will leave a short book review even if no one else wants to read it with me.
I think having a less prescriptively "parenting" focused book might actually be good for helping inform us rather than making us feel guilty that we might not be doing everything exactly right. Mostly because people seem frustrated with Unconditional Parenting being so negative for so much of the book (I know I was).