Also available: School of Velocity Op 411 Book 2
With so many study books around, most of which contain music by Popp, you may find it puzzling to find that I have included these rather expensive items in my list as well. Rather like the Lambert volume though, they do contain some delightful studies. They are all short, grouped together in keys and each one addresses a specific problem. They are also tuneful. Items for the teaching bag I think.
Difficulty level, roughly compared to ABRSM exam grades. 0 is beginner, 9 is advanced (beyond grade 8).