My local music store recommended The Complete Book of Alternate Tunings by Mark Hanson. $17.95 US. ISBN 0-936799-13-7.
Though I'm usually wary of such recommendations, I ordered it anyway, and was pleasantly surprised. Among my personal discoveries was the beauty of the Double Drop D tuning, which is perfect for the band I play in... lots of tunes in D, G and A, yet other keys still easily playable by emphasizing the middle four strings.
I notice that Mark has published another book titled Alternate Tunings Picture Chords. Does anyone have a quick opinion or review of that one?