John,
It depends on your area of interest and funds available.
Sean,
In the US, I would contact Ron Muszalski (a personal friend of mine, and a wealth of info), Peter Yagi up near Seattle, Dick Donovan, although I don't know how much selling he is doing these days, George Lewis I believe closed up his business. Rhod McEwan puts out far and away the best catalogue. Pacific Book Auctions is a great place to look for golf books too if you can show up in person in San Francisco, because they sell books by lot (can be 5-20 books or so) at the end of the auction and is a good place to start a collection.
Also, I will have many books for sale in the coming year as I try to finance a golf trip to Australia, so have your friend get in touch and I'll be happy to help.
BTW, be careful of Jon Bonjenoor as I have heard sketchy things about him.