I can't imagine Gordon Ramsey sitting down at a computer, can you?
Hugh Fearnley-Whittingstall, I suspect, writes every word of his, because the River Cottage Meat Book reads like exactly the kind of cookery book someone with an Oxford arts degree would write.
Jamie Oliver - I think he wrote the first Naked Chef book and the next few were done on a dictaphone. Now someone just has a chat with him and produces Jamie-like text, just like someone writing a celebrity biography, though I'm sure he does most of the recipes (that's my guess).
Does anyone actually know, or want to join me in speculating?