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I have heard about Popper, but never seen his books anywhere. I did quick search and it looks like they are not easily available in my country.
I have two books.
Dotzauer: Violoncello-schule Vol I and II.
Vol I is suitable for beginners. All etudes are in first position. They are very easy in beginning and gradually become more difficult.
Vol II introduces positions from second to fifth. One by one first and then all together.
I’m allmost in the end of book 2 now. I like both of these very much. Most of etudes are interesting to play because they sound like ”real music”, not just some practicepieces. Many of them are also duets and can be played with teacher.
I go back to book 1 sometimes when I want to practice some bowing techniques, etudes there are so easy that I don’t need to think my left hand fingers and can focus 100% to my right hand.