A new Blender book has just been released by Packt Publishing called "Blender 2.49 Scripting" (by Michel J. Anders). I'll be reviewing the book after I receive it and have had time to read through it.
I was a little bit hesitant at first because its focus is on Blender 2.49. However, when you consider that the Blender 2.5/2.6 Python API has quite a ways to go before it's as good as the Blender 2.49 implementation, it may be more practical to get up to speed with Blender 2.49 Python scripting.
There's a great deal of extra power and control that can be introduced into Blender via Python scripting and it's nice to see a book dedicated to the subject. I'm sure there will be an edition released for Blender 2.5/2.6 once it becomes stabilized.
Here's a sample chapter from the book "Chapter 5: Acting on Frame Changes"