It’s probably pretty hard to find generically ‘useful’ books for TA’s- well, for CG in general. There are some bona-fide must haves, like ‘anatomy lessons from the great masters,’ some sort of 3D math textbook, etc., but generally technical stuff is usually too simple, too outdated, or too specific.
If you’re looking for stimulating learning materials, I tend to go the route that variety enhances ability and creativity… Read history to better understand people, read biography to learn more leadership, read about technology to improve problem solving, read about anything that interests you. Watching any tutorial DVD or reading any book, I feel I am learning, improving myself- likewise, I know that when we work all day and make no progress we can hold our head up because ‘at least we learned something.’ I find whenever I enjoy and learn more about a subject it always influences my way of thinking about something, and the more ways of thinking about something the better equipped you will be to handle it.