Couple of books that you might like ( they are mostly for dress/fashion but you can apply that to softgoods)
"How to Use, Adapt, and Design Sewing Patterns: From store-bought patterns to drafting your own: a complete guide to fashion sewing with confidence "
"Pattern Making (Portfolio Skills)"
"Couture Sewing Techniques" is another book I can recommend, it is less about patterns and more about hand sewing technique, but it helps with terms and there are a few others specific books to bags and whatnot.
This site has also proven very handy:https://blog.colettehq.com/projects/bag ... -resources
Cheers! I am actually off to do some patterns