The ruling bodies of Golf:

An excellent discussion forum on the Rules of Golf is hosted by a friend of this site,
The Leith Society

Probably the best book for an insight into the development of the Rules of Golf from its earliest times,

Rules of the Green, by Ken Chapman

If you have an interest in the history of golf, you'll enjoy this really superb golf history site:

Scottish Golf History

And for almost everything else you would ever want to find on golf, visit

Thor's Golf links