I had a similar occurrence, this past summer it got extremely hot and humid here in Florida, more than it usually does. My Tokonole looked like a curdled dairy product. And it smelled odd. Very close to your picture. I would bet it encountered extreme temp change.
The resources topic includes books and other sources for all kinds of things for leather crafting. I think you are right it should have its own area on the forum. Thanks for sharing this book.
I find simple is easier. I use regular printer paper to draw or copy the design on. Then just case(wet) the leather. Set the pattern in position on the leather where you need it to be. Use painters tape to secure it from moving, then I use a regular pencil to follow the pattern, imprinting the design into the leather . Use a practice piece first to get the "feel for how much pressure to use to get a good impression. Then swivel knife away! Happy leather crafting!
For the purse 4-5 oz is plenty, if you need it to be stiffer add a bag stiffner . The wallet needs to be light or you will be skiving all day, imo. 3-4 or 4-5 oz back,and 2-3oz pockets. Only my opinion. good luck and have a great new year.