(listed in order of difficulty from easiest to hardest)
Elementary Method for Clarinet - Hovey
Rhythms Complete - Bugs Bower/Charles Colin
Selected Duets for Saxophone, Vol. 1 - Voxman (editor)
Selected Duets for Saxophone, Vol. 2 - Voxman (editor)
Selected Duets for Clarinet, Vol. 1 - Voxman
Selected Duets for Clarinet, Vol. 2 - Voxman
Selected Duets for Flute, Vol. 1 - Voxman
Selected Duets for Flute, Vol. 2 - Voxman
Reading Studies for Guitar - Leavitt
Advanced Reading Studies for Guitar - Leavitt
Easy Classics to Moderns (for piano - read right hand part)
Vol. 17, "Music for Millions" series
25 Etudes for Guitar - Carcassi
Free PDF
Etudes for Violin - Wolfhart
Free PDF
Violin Duos - Bela Bartok
Free PDF
Jazz-Rock Rhythms - Charles Colin
Bop Duets - Bugs Bower/Charles Colin
Advanced Rhythms - Joe Allard
Jazz Conception for Saxophone - Lennie Niehaus
Saxophone Duets - Lennie Niehaus
The Technique of the Saxophone - "Rhythm Studies" - Joe Viola
The Technique of the Saxophone - "Chord Studies" - Joe Viola
Develop Sight Reading - Charles Colin (publisher)
Classical Studies for Clarinet - Voxman (editor)
A Guide to Jazz Phrasing and Interpretation - John LaPorta
Rhythmic Articulation - P. Bona
Free Public Domain Scores - Thousands of scores. Requires some digging to locate treble clef parts in the playable range of the guitar. Try searching for guitar, violin, clarinet, flute or piano (right hand) music.
Thanks to Greg Arney at HubGuitar for providing links for many of these books.