I’ve just started reading The Writer’s Journey: Mythic Structure for Writers, by Christopher Vogler. It came quite highly recommended.
Like most (if not all) writers, I’ve read a goodly pile of books on how to write. Some of them were more helpful than others and I’ve put a list of the ones I use as reference in the right-hand column.
Boudica Marginalia 
Oh! And don't forget Stephen King's `On Writing!`. It's an all-time favourite of mine…lol x
Everyone raves about it, but I haven't actually read it so I can't add it to the list. One day I'll get around to it!