I highly recommend Jean Ritchie's book and also that of Larkin Bryant. Both explain in clear and easy to understand terms the different modes, tuning, finger techniques, etc. as well as have sheet music. Jean's of course, gives a lot of history with wonderful photos from her family's early days.

I agree though what I love about the instrument is it is so wide open to what YOU want to do with it..tuning-wise, strumming ( I mostly strum away), etc. Also, don't be afraid to try different kinds of picks, even feathers, like the traditional goose feather.

I second that about DAC (Aeolian mode), I love its minor tones, but I know more tunes that work in DAA (Ionian mode) so, I guess I'd better practice come new ones!:-)