Start with any beginner book, Mel bay, Alfred, etc, Vol.1; they have all the answers you will need for a while. Doesn't even have to be classical.
Lester Devoe Flamenco Negra
Lester Devoe Flamenco Blanca
Aparicio Flamenco Blanca with RMC pickup
Bartolex 7-string with RMC pickup
Giannini 7-string with Shadow pickup
Sal Pace 7-string archtop