I suspect it may be overkill, but I've been to IMSLP, and there are quite a number of Public Domain guitar methods published there. There's several editions of Sor and Coste, Aguado, Carcassi, Carulli, Giuliani, etc, and even
IMSLP112274-PMLP229290-ribeiro_nova_arte_de_viola.pdf
(Portuguese: the Portuguese refer to the guitar as the viola or violao, and a six-string cittern tuned in open C - CEGceg - as guitarra: Ribeiro Nova gives training on playing a five-string guitar.)