The four books listed here are the ones that I have found particularly useful during the research for this program.  Obviously there are other good books in the music field, but these are the ones I considered myself lucky to have stumbled upon.

Hearing and Writing Music - Professional Training for Today's Musician, by Ron Gorow, (c) 2000, 2002 Ron Gorow

Written for composers, orchestrators, arrangers and performers, this book is highly educational -- and enjoyable to read -- for musicians of all levels.  Possibly the best book in its category and one that covers a broad spectrum of subjects related to hearing and writing music, plus the most clear presentation on chord symbols conventions, which is reprinted by permission in this manual.  See About Chord Symbols.

When FastChords "guesses" the name of a chord, it uses the chord symbol conventions advanced in this book.

Contemporary Music Theory - A Complete Harmony and Theory Method for the Pop & Jazz Musician, by Mark Harrison, (c) 1995, 2001 Hal Leonard Corporation.

There are three books in this series: Level One, Two and Three.  Level One is a very clear and complete introduction to music and chords theory.  The series  goes on from there to more advanced chord harmony concepts in levels Two and Three.

The New Harmony Book - A Musical Workbook for Classical, Rock, Pop and Jazz Music, by Frank Haunschild, (c) 1988, 1994, 1997 AMA Verlag GmbH.

Translated from German, this is an excellent but hard to find book.  Great intro to music theory plus interesting chapters on contemporary scales.