You need to get
The Human Figure by John Vanderpoel and 'Bridgman's Complete Guide to Figure Drawing' by George Bridgman. Both are solid time tested texts on drawing.
Also recommend anything you can find by Andrew Loomis. The only thing in print by Loomis is a Walter Foster book called 'Heads 2' (The name changes with each reprint). Look for Loomis at used book sites--caution, his books are expensive if you can find them.
Harley Brown recently released an excellent book called
Eternal Truths For Every Artist.
Finally, the most important thing you can do is buy a skull and learn it. Learn its simple relationships and nuances so that you can see them under a person's flesh. Nothing will catapult your skill more than this.