Jack Kearney's "Tracking: A blueprint for learning how"..
The text is focused on man tracking.. however, when combined with a study of animal behavior, the tracking principles outlined in the book apply just as well. Both types of tracking are things that I'm interested in, and practice, and really there isn't much difference.
I've read a ton of books on both animal and man tracking. I've also attended more than my fair share of tracking training sessions.. After all this, I've come to the conclusion that Kearney's book was the most helpful in the learning process.
It is exactly what it says it is.. "A blueprint for learning how". The book outlines lessons that can be done by yourself.. you progress through the chapters in order, and by the end, you'll have gotten a ton of dirt time, and a great deal of knowledge on how to actually track.
After that.. I'd use some of Tom Browns books as supplements, and as a means to improve specific parts of the skill. Brown's "The Art and Science of Tracking" does a pretty decent job of explaining pressure releases and the like.. and this information has helped me increase my skill as a tracker.
In the end though. "Tracking: A Blueprint for Learning How" has been the one that sticks out to me as the best for actually learning about the process. Unfortunately.. you may have a hard time finding it..
www.nasar.org sells it, so do a few of the tracking services out there.