20 Studies for the Guitar, by Fernando Sor, edited by Segovia (book + CD).

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)