I've been reading about hand stitching leather for a while now, and after hearing so many people recommend Al Stohlman's The Art of Hand Sewing Leather (1977) on Leatherworker.net, I bought it.
It's full of great illustrations and lots of weird emphatic old-man ellipses. A compact wealth of knowledge, truly.
This is one of the best resources for American Western-style hand stitching. I haven't looked too much into other styles, but Nigel at Armitage Leather has a great YouTube channel if you're interested in the English tradition.