Here are some from the FAQ were putting together:

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• Pumping Nylon by Scott Tennant
• Parkening Method by Christopher Parkening
• Aaron Shearer Method
• Library of guitar Classics
• Estudios Sencillos (simple studies) by Leo Brouwer - PDF version
• Twenty Studies for the Guitar, by Fernando Sor arr. Andrés Segovia
• Diatonic Major and Minor Segovia scales - PDF version
• Sight-reading for the Classical Guitar by Robert Benedict
• The Bible of Classical Guitar Technique, by Hubert Kappel
• Kitharologus by Ricardo Iznaola
• Books by Charles Duncan
• Books by Christopher Parkening
• Any one of several from Frederick Noad

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I would personally reccomend pumping nylon/kitharologus for getting better at technique and parkening/estudios/library of classics for pieces