I just finished (again) David Dodd's The Complete Annotated Grateful dead Lyrics. I find it most interesting; some the annotations are quite insightful (and some, to be sure, are pure, unadulterated bovine waste material......).
But it is well worth picking up just for Robert Hunter's Foreword and John Barlow's Afterward; wonderful glimpses into how these two giants viewed their art and their place both in the band and in history.
Peter