The Hollywood Animation Archive has posted a complete copy of an excellent book on animation. It's called 'Advanced Animation' and it was written by Preston Blair in 1947. Blair used the characters he had been working on for Disney and MGM to illustrate his examples. He goes through all the requirements for animating, from drawing heads, hands, and complete characters to animating walks, runs, dialog and emotions. It's amazing. You can follow along and draw all the sketches yourself and you end up having taken a complete course in animation!
Go see it here.