Big Al,
The clues are all in the tunings... baffled me for years. However look no further than the "Nick Drake Song Collection "..a song book.. try Amazon..Some clever chap has worked them all out, and believe me the open tunings open up the eye to some wonderful colourings and textures, and of course the secrets behind playing the songs.. From a guitarist point of view, it helps with the left hand (If you are right handed)some , once worked out are surprisingly simple... But it can't help with the right hand .....Drake could have seperate rhythms going at the same time with his right hand.No book will ever fathom that ...so....Good luck