Feynman has a fantastic anecdote about getting over what we might today call burnout. He was not interested in work at all in his professorship, when he noticed a wobble in a plate being thrown, and wondered about it. He spent all day working on the physics of this toy problem, and claims discovering play as the key to his recovery.

If you haven't read them yet, "Surely You're Joking, Mr. Feynman", and "What Do You Care What Other People Think" were great reads in my 20s.