Essays. I read them online (and commented there, too, the Mary Catelli cited in one is indeed me), and now in dead tree form. Plus a new essay exclusively for the books.
Most of the examples are drawn from fantasy, but may be of more general interest. The title essay discusses what, exactly, is the significance of style in fiction. And the others touch on ways to write, inspiring effects of other fiction, taking apart Heinlein's Rules, and more.