I marked 'learned the hard way, no help' because that's pretty much how it is. I'm just beginning, and other than books, I don't really have anyone who is working in leather doing what I'm doing.
However I do get some much valued assistance from my husband, who may know little about leather but knows a lot about pattern making & sewing as he was a parachute rigger in the Navy & learned also to design & make his own utilitarian clothing. I also have a longtime friend in California who has had an international business for many years, building soft shell musical instrument cases out of leather or cordura. He admits that his work is 'carpentry' not 'aesthetic' but his designs are nonetheless elegant as well as functional. I can email him or call him & he will attempt to answer my newbie questions.
Still, all in all, I'm struggling along on my own. I could really use some classes on pattern making, how to line up lacing holes better, what hardware to use where, etc. etc.! I do feel lucky though, that I'm already an artist & have a small established business making & selling bead jewelry & handmade glass beads that I make, and this has given me the confidence to go ahead and branch out into leatherwork, to which I was exposed decades ago and always wanted to study!
I was thrilled to find this forum & hope I can find answers to some of my ongoing questions here, too. Clearly there are some fantastic artisans here, whose work will be very inspiring to me.