I think Peter Main's book on coloring is a bit more up to date with coloring techniques. Al's books are still good to have for basics and fundamentals.
Here's a list:
The Main Technique of Coloring Leather - Peter Main, available from Peter directly or through Hidecrafter
How to Color Leather - Al Stohlman, available at Tandy, Hidecrafter, et al..
Coloring Leather - Al Stohlman, available at Tandy, Hidecrafter, et al..
Marlon