Green's Functions for Solid State Physicists

I liked this book a lot because it teaches this stuff through examples of when some seemingly abstract quantities are actually observables, but this approach is not always as lucid as a formal text because the general results need sometimes be distilled from the examples. I think it makes a great companion (esp. since it's not terribly expensive) to like a Fetter/Walecka or Abrikosov et al. or whatever more general text you like.