I hadn't realized it till KTC said it, but I also pick the nuggets out of writing books. If you get one or a few that resonate for you that makes it for me a worthwhile read.

I also agree with what Harlequin said, in that I get a lot out of seeing principles in action.

Besides On Writing, also Becoming a Novelist by John Gardner. And the Mailer book, The Spooky Art is probably more an account of one writer's journey, but for me there's affirmation to be found in those kinds of books as well.