Agree with Michael Stickylearning Eury on branding and on those books. Oddly enough, I find branding books (marketing approx. equal to behaviour change) useful for practice too.

My 2c on the books: Selling the Invisible by Harry Beckwith. Seriously good for consultants on service marketing. I think I've bought this book a dozen times now for gifts. Seems almost patronisingly basic but every page has something meaty (if you can stand having McDonalds being touted as the ultimate in service marketing...)