I'll always say Digges as it was the first beekeeping book (given to me by my grandfather when I was nine) I read, and when you look at the times the book was reprinted from 1902 until 2002, it tells you a thing or two.
Manley, Snelgrove, then later, Hooper and de Bruin are must reads IMHO. I read Cramp's book on beekeeping many years ago when I was really getting into my beekeeping and promptly left him on the shelf with my other 'doubt I'll read again' books. Of course with my interest in African beekeeping there's also Francis Smith and May's 'Beekeeping in Southern Africa'(who doesn't tell us a lot, but it was a start)
There are loads of others I've hoarded over the years, many that I'm still getting to grips with.