Each volume contains 25 of the 50 etudes designed to improve the musician's technique. Book one starts at beginner level and progresses to early intermediate, while book two picks up there and progresses to late intermediate. In general the etudes stay with the more common keys, but cover a full range of musical challenge.
These are tuneful pieces, not purely technical, although each advances a particular technical idea. Overall there is a lot of interesting, playable material here for the developing instrumentalist.