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

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The Renaissance Guitar, by Frederick Noad. A great collection.

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The Norton/Grove Concise Encyclopedia of Music

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Learning the Classic Guitar, Vols. 1-3 (© 1990), by Aaron Shearer.

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Art of Classical Guitar Playing, by Charles Duncan.

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Villa-Lobos Solo Guitar, Collected Works, edited by Frederick Noad.

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The Guitar and Its Music: From the Renaissance to the Classical Era
By James Tyler and Paul Sparks, Oxford University Press (2007)

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