It gets a bit technical (at times it reads like a physics textbook!), but "The Art of Classical Guitar Playing" by Charles Duncan gives the most in-depth coverage I've found on right and left hand technique, nail filing, articulation, coordination, building speed, etc. Not the most entertaining reading, but certainly comprehensive. Note: although has a few snippets of music here and there, this book does NOT generally include exercises and studies to be played - it simply describes techniques using (mostly) words, supported by diagrams and photographs. I would recommend this book especially if you don't have access to a teacher who would normally be able to advise you on the more mechanical aspects of technique. Also, as mentioned previously, Pumping Nylon is an excellent book.