The DADGAD bible is Pierre Bensusans' "The guitar book" He has developed a whole method for that tuning.The book has full transcriptions of many traditional tunes as played by Bensusan with full tabalature and notatation plus finger exercises etc.I think of DADGAD as standard tuning I've been using it as standard for about 14 years now.Richard Thompson also produced a good book and CD rom tuition set that has some nice DADGAD tunes.I also use DGDDAE or CFCCGD if I want that loose feel.It doesn't take long to get a grip of DADGAD but it requires a bit of application to revisualise the fretboard so that you can transfer what you've learnt in conventional tuning into fluency in DADGAD.