Thats an extremely vague question.
What have you made so far?
What do you plan to make in the future?
What do you want to learn:
Plaiting/braiding?
Carving and tooling
Molding?
Stitching?
Lacing?
I can only really recommend the plaiting side. Bruce grant's encyclopedia of rawhide and leather braiding will give you everything you need to know to keep you going for a long time...
Wanna expand into whips, try Ron edwards How to make whips, second edition. I just picked this up and it covers all the basics and helps the advanced too.
Want stitching old school and happen to cross, Al sthohlmans guides, I got mine for about 10 pence each, very detailed but not at the current market prices as they are rarely more than 20 pages.
Try a trip over to leatherworker.net for some great people to talk to.