Richard Cook's Jazz Encyclopedia is remarkably comprehensive and one of the best biographical jazz reference books I've come across.
My only minor complaint is the omission of an entry for the late, perceptive jazz writer and broadcaster, Charles Fox. Unusually, for a critic, he was well respected by musicians and his superb, long running 'Jazz Today' programme on Radio 3 was very influential. Charles contributed enormously to jazz appreciation in Britain and, hopefully, he will be included in the next edition.
Apart from that I'd thoroughly recommend this wide-ranging encyclopedia which should be an essential part of any jazz library.