These delightful etudes by Ernesto Kohler (Op.33) have endless benefits for an intermediate player.
Each one succinctly addresses a particular idea - be it articulation, phrasing, or an expressive gesture - consistently throughout the etude. Thus their intention is focused and, due to their brevity, a student can be assigned two to four a week. Consequently, the rewards are great!
Book 1 contains exercises No. 1 to 15.