The index of French horn music / compiled and edited by Wayne Wilkins.
Magnolia, Ark. : Music Register, c1978.
MUSIC Ref ML128.H67 W5 and supplement
Horn Bibliographie / Bernhard Brüchle.
Wilhelmshaven : Heinrichshofen, c1970.
MUSIC Ref ML128.H67 B8
The horn : a comprehensive guide to the modern instrument & its music / by Robin Gregory.
New York : Praeger, 1969.
MUSIC ML955.G7 1969b
Horn technique / Gunther Schuller. 2nd ed.
Oxford ; New York : Oxford University Press, 1992.
MUSIC MT420.S35 1992 c.2
University of Iowa music source book : wind and percussion materials / [faculty members of the University of Iowa School of Music]
[Iowa City] : Paul Anderson, 1986
MUSIC Ref ML128.W5 I48 1986x
The horn call , v. 7 (1976)- ML1.H617
The horn call annual : refereed journal of the International Horn Society, no. 1 (1989) - 9 (1997) ML1.H6172
Brass bulletin--the Library has selected volumes between 1983 and 2003 ML5.B6864
M - Scores
M80-84--Solo horn music
M255-259--Music for horn and piano
M1028-1029, M1128-1129--Horn with orchestra and string orchestra (includes full scores and solos with piano arrangements)
ML - Music Literature
ML128.H67--Horn music: Bibliography
ML955-958--Books about the horn
MT - Instruction and Study
MT420-432--Methods, etudes, orchestra studies, etc.
Horn and piano music
Concertos (Horn with string orchestra)
Horn with orchestra
...and there are lots more. Do a subject search for "horn" and see the 63 different subject headings that begin with "horn" and are about the French horn.
Horn (Musical instrument)--History
... and lots more. Search "horn" in Encore and find more than 700 recordings and more than 1100 scores.