Mackenzie's "Goat Husbandry", 5th edition. May be thin on the ground in 2ndhand shops/abebooks right now. I think the 4th edition is probably quite good as well if you disregard the veterinary medicine bit, for diseaes/vet care etc, I would recommend David Harwood's "The veterinary guide to goat health and welfare", available from the BGS, but probably only to BGS members. I also quite like Peter Dunn's veterinary goat book. There is also a lot of info on the BGS website.