For dadgad, I have a tape of Don Ross that covers some great material that I would recommend. Info
http://www.gobyfish.com/
Another real gem, although not a song book per say is "The Complete Book Of Alternate Tunings" by Mark Hanson. It has all the different tunings, artist lists with songs played in which tuning, what cord patterns are available in each tuning. I also has a section of songs that are frequently played in alternate tuning when the song was written in standard tuning.