This happens to me a lot - I'm happily reading one book, another catches my eye, and before I know it I'm reading 5 books at a time.
Book Number One:
The First Idea: How symbols, language, and intelligence evolved from our primate ancestors to modern humans. Stanley I Greenspan and Stuart G Shanker. This has it all - sociobiology, emotions & the development of intelligence, and what looks like a good reason to read the whole thing, the final chapter entitled, "Future Evolution: Toward a Psychology of Global Interdependency."
Book Number Two:
The Tao of Equus: A woman's journey of healing and transformation through the Way of the Horse. This book is simply gorgeous, deep, complex and purely simple at the same time. There is a profound synergy between this book and book number one that bounces through my brain and body with delicious energy.
The Book I Haven't Gotten to Yet, that if I do get will probably trump all of these:
Animals Make Us Human: creating the best life for animals. Temple Grandin.