Building and Painting Scale Figures by Sheperd Paine.
1995 Kalmbach Books
Unfortunately this book is out of print. It does become available through used book dealers and is well worth the effort. Even though it is written for the large scale military modeler it has extensive information that applies to all scales. The "Stop sign rule" taught by Mr. Paine is widely accepted as the foundation for shading and highlighting for overhead lighting.

How to Build Dioramas by Sheperd Paine
2000 Kalmbach Books  
Chapter four, Posing and Painting Figures contains a lot of Paine's information from the "Painting Scale Figures" book. As the title suggests there are a lot tips for those interested in vignettes, dioramas and shadowboxes.

Bill Horan's Military Modeling Masterclass by Bill Horan
1998 Windrow & Greene Publishing
This book deals with building and painting figures in the 56-120mm range but many of the techniques carry over to smaller scales. Horan's work is inspiring!

Euro Modelismo Monographs
Painting: Overhead Lighting text by Rodrigo Hernandez Chacon
Painting: Overhead Lighting II text by Rodrigo Hernandez Chacon
Accion Press
The greatest value to these two volumes is that they are written for painters using acrylics. Some good information and illustrations. I find the witting to be cumbersome. Color charts accompany the figure painting examples.