I have a lot of favorite writers, but for one who is instructional I have to choose Juliette Aristides and all three of her books on art.  It should be required reading for all photographers even if you have no intention of being an artist it will make you a better photographer.

Each paragraph is so beautifully written and so intelligent, she never glosses over a subject and at the same time you are fully informed, her chapter on composition was the best I had ever read.  I love this writer and her books.