Adobe After Effects CS4 Classroom in a Book by the Adobe Creative Team.
The CIB series has always been a good investment of time and money. This edition is no different. If you're looking to learn After Effects, this is where you need to get started.
There are 14 lessons covering all aspects of After Effects starting with workflow and including masking, keying, 3D features and other basics. I really like that they introduced the tracking features. This is not a tool for beginners but the Adobe Creative Team has made it simple.
One caveat. This book will not make you a professional After Effects user. The only thing that will do that is practice, lots of it, But this book is what you need to get started.