M - Scores
M125-129--Solo guitar music
M276-277--Music for guitar and piano
M292-293--Music for two plucked instruments
M1037.4.G8, M1137.4.G8--Guitar with orchestra or string orchestra (includes full scores and solos with piano arrangements)
ML - Music Literature
ML128.G8--Guitar music: Bibliography
ML1015.G9--Books about the guitar
MT - Instruction and Study
MT580-599--Methods, etudes, orchestra studies, etc.
Guitar music (Guitars (2))
Concertos (Guitar with string orchestra)
Guitar with orchestra
...and there are lots more. Do a subject search for "guitar" and see the 120 different subject headings that begin with "guitar."
... and lots more. Search "guitar" in Encore and find more than 150 books, more than 550 recordings and more than 250 scores.