Cello Forum David Popper: An Anniversary Retrospective Part II: His Legacy as Performer and Pedagogue

This may have to do with the fact that they were originally published in separate volumes, beginning in 1901 and continuing until 1905. The Piatti Méhode de Violoncelle, for example, progresses very logically, beginning from a rudimentary level and concluding with studies that require a high technical command. Unfortunately, to my knowledge, it is out of print but can still be found in some libraries.