The use of different feet on your machine is mainly common sense. You use a welt foot if you're sewing welts, and a presser foot if you're sewing regular straight seams. You use the top stitch feet when you're top stitching, and the single right feet when you need to avoid an obstruction to the left of the needle that a regular foot would hit. I really know of no book that would help you, it's just experience and repetition.
There really are no different stitches, they are all the same straight stitches. Some are bigger than others or are placed differently, but they are all still straight stitches. If you know how to top stitch, you know how to make a French seam.
The only places I buy leather from only sell to businesses, so that wouldn't help you, but I suggest you learn more about leather and what makes one hide good quality and another one poor quality. There are huge differences, and you should know what they are the next time you buy. I suggest you look at this link from Spinneybeck: CLICK HERE
Spinneybeck sells fabulous leather, but it's really expensive. Also Google "leather" and look at the Wikipedia entry for it.