The Renaissance Guitar, by Frederick Noad. A great collection.
The Norton/Grove Concise Encyclopedia of Music
Learning the Classic Guitar, Vols. 1-3 (© 1990), by Aaron Shearer.
Art of Classical Guitar Playing, by Charles Duncan.
Villa-Lobos Solo Guitar, Collected Works, edited by Frederick Noad.
The Guitar and Its Music: From the Renaissance to the Classical Era
By James Tyler and Paul Sparks, Oxford University Press (2007)