I've got a few but the Haynes manual is the easiest to understand, I reckon. It also has brilliant photos which really helps. I do like Bees at the bottom of the garden which came recommended on my course but the photos and diagrams are not nearly as helpful as the Haynes and I don't find it quite as comprehensive. I read the Haynes first, understood parts of it, went on the course, went back to the book and it all made a whole lot more sense and also helped clarify the course.
H