1. Popper High School of Cello Playing - These 40 studies are simultaneously the king of all etudes and the bane of every cellist's existence. Janos Starker once said that there is no technical problem that Popper cannot solve. These studies are often grueling and complex, but address every technical aspect from specific bow strokes to various left hand position patterns. These etudes help prepare students to play the most difficult passages in cello concertos, virtuosic show pieces, and other advanced repertoire.
2. Piatti 12 Caprices - Piatti's caprices are both incredibly valuable for learning advanced techniques as well as performable as short concert pieces. I recommend purchasing the Barenreiter edition because is comes with an appendage book that includes information about each etude and a list of the standard cello repertoire in which the particular technique can be found.
3. Duport Twenty-One Etudes for the Violoncello - Duport's etudes are another staple of cello pedagogical repertoire that also function as great concert pieces. My favorite thing about these etudes is that they are written as duets that can be performed with another cellist.