For me this is one of the best ever, and great that its totaly free online.

I think Jack Hamm's Drawing The Head And Figure is better in many areas, but not public domain.

All the books mentioned in the list are amongst my favourites. Amazing that after all this time the oldies are the best. Probably because modernists took over the art establishment and ruined teaching traditional drawing skills.

The Bridgeman books are fantastic too. Great list of wonderfull free books :)