Mastering Jumping Skills 1 is an overall introduction to jumping; includes background info about the jumps themselves, on the different jumping skills, leads, etc It includes fitness & conditioning for jumping, foundation flatwork, intro to jumping and 1- and 2- jump skills.
Volume 2 picks up where 1 left off... beginning sequencing, adding handling skills to the mix, chapters with drills on the specific skills (collection, extension, slice, landing-side approach, etc), Jumping Problems, early takeoffs, etc etc.
So, they cover different material. The publisher tells me Volume 2 is on their radar for early 2019...so I’m hoping it won’t be much longer until its published.