Robot Builder's Bonanza, 3rd Ed
A must have for any entry level robotics hobbyist. Covers much of the basics, including sensors, controllers, batteries, and mechanical construction.
Society of Mind
Written by Marvin Minsky, a father of AI. Great primer for those interested in Artificial Intelligence. He takes the approach that
the brain is not just one computing unit, but actually a 'society' of many many different computing units working together.
Great for building manufacturing robots. Intermediate level. Useful to learn how to build your robot cheaper and faster (not how to build a robot). If one day
you plan to manufacture and market your own robot product, this is a very valuable read