So just reference drawing for models? Honestly, lots of practice and good reference images will probably get you most of the way to where you want to go. It doesn't sound like you want to do full scenes, which gets considerably more complicated. You will definitely want to look at anatomy and human/animal proportions if you're talking about characters (and consider getting a mannequin like Sunandshadow suggested). Tracing is okay but don't let it become a crutch.
As for a book, this is a good one with many game examples: http://www.amazon.com/Drawing-Basics-Video-Game-Art/dp/0823098478
I also wrote an article which will at least give you some vocabulary so you can narrow your search for resources: