I'd say that Milan Kundera has produced the best, most complete testimonial on the craft of novel writing among any of our contemporaries. Recently my favorite of his essays was The Curtain. But in his brilliant The Art of the Novel, he indicates that metaphors should be used sparingly in fiction. Those metaphors that are used should be of the utmost power, and of central importance to the book.
For a long time I disagreed with him, but now I see his point. As I shop my first novel Love Song of Zero and One to agents and independent presses, I have begun writing my next called Alchemy of Air. I'm not claiming to be extremely discreet about my use of metaphor in this book, but I have attempted to answer Kundera's call for caution, and respond to his championship of the novelistic essay in Alchemy.
Also, Kundera's "nonfiction" books should be considered by anyone who loves the Art of... series by Graywolf, a great edition of which by Dean Young called The Art of Recklessness I reviewed here awhile back.