As a supplement to music reading, I like to use the Applebaum series called “Beautiful Music for two String Instruments.” It is available for violin, viola, cello or bass. Any combination of instruments can be used. The books also have piano accompaniment available. The duets sound very good with the upper strings on melody and the lower strings on the harmony lines. I find the second parts especially useful for the kind of parts cellists play.
The pieces have the key signature written at top of the piece. A number of different key signatures help students practice a variety of key signatures, however, it stays away from very difficult keys for the instruments.
I did find it disappointing that there are a number of errors in the viola and cello books. I plan to contact Alfred Publishing Company to let them know. I don’t know if they would be able to make corrections, but there are actually wrong notes in the newest edition of the viola and cello parts. I will post an update if I get a response.
My favorite of all the pieces is the Mazas piece at the beginning of the third book. It is significantly more complex than duets in the previous books.