I would say that if you simply want to rebuild your stock Type 1 Beetle/bus engine, get yourself a copy of Tom Wilsons "How to rebuild your volkswagen Air Cooled Engine" together with a copy of the Bentley and/or the haynes manuals.
If you want to get into tuning, then as Rich say's there are some good books available by Mr Seume and Bob Fisher (Although some of this is a bit dated now). And some of the stuff on the net is good too, but be selective.
But above and beyond all that, you can't beat talking to people face to face who have done the job themselves.