The index of viola music / compiled and edited by Wayne Wilkins.
Magnolia, Ark. : Music Register, 1976, and supplements to 1978
MUSIC Ref ML128.V36 W5
Literatur für Viola / Franz Zeyringer
Hartberg : Schönwetter, 1976
MUSIC Ref ML128.V36 Z5 1976
Music for viola / Michael D. Williams
Detroit : Information Coordinators, 1979
MUSIC Ref ML128.V36 W53
String music in print / by Margaret K. Farish
New York : R. R. Bowker Co., 1965, and supplements to 1984
MUSIC Ref ML128.S7 F4
Journal of the American Viola Society, v. 3 (1987)-v. 10 (2001) ML900.V62
Die Viola : Jahrbuch der Internationalen Viola-Forschungsgesellschaft, 1979-1984 ML5.V564x
American string teacher, v. 8 (1958)- ML27.U5A8356
The Strad, 1991- ML749.5 S773x
M - Scores
M45-49--Solo viola music
M224-228--Music for viola and piano
M1014-1015, M1114-1115--Viola with orchestra and string orchestra (includes full scores and solos with piano arrangements)
ML - Music Literature
ML128.V36--Viola music: Bibliography
ML900-905--Books about the viola
MT - Instruction and Study
MT280-298--Methods, etudes, orchestra studies, etc.
Viola and piano music
Concertos (Viola with string orchestra)
Viola with orchestra
...and there are lots more. Do a subject search for "viola" and see the 144 different subject headings that begin with "viola."
... and lots more. Search "viola" in Encore and find more than 250 books, more than 1000 recordings and more than 1500 scores.