SparkFun has a lot of useful products, especially sensors. They also have a lot of good tutorials. They have just started up a video series on concepts in robots. A few minutes each, they do a good job of explaining basic concepts. The first video is a bit of a promo of their series and new products, but later videos concentrate on the concepts more. I recommend that you go to the intro video, and subscribe.
Transfer of Motion
As always, SparkFun has an extensive Learn section on their website, with tutorials and curricula available.
Mechanical Principles (1930) * by Ralph Steiner