Another vote for Our Babies, Ourselves (Meredith Small). Really helps to tease out what is purely cultural about child-rearing, allowing parents to make informed choices about which bits of culture to accept and to reject. Very well-reasearched!
Dr. Sears' Baby Book is a great reference and primer. Loving Mama is a nice book of essays, but better for the mama who is already doing AP -- might be too heavy-handed if someone is undecided.
The Contiuum Concept. Not ABOUT attachment parenting, per se, but reading that book led me to AP. It's more about instincts, community, and natural parenting (it talks about babywearing, co-sleeping, etc)... and it is THE BEST BOOK I've EVER read about parenting. Seriously. I give a copy to every pregnant woman I know.