"Winning Poker Systems" by Norman Zadeh is very good, though you'll have to find a used copy I think. Only part of the book covers draw but that section is, IMO, excellent. Malmuth's "Winning Concepts in Draw and Lowball" is good too - a little more conceptual and less mathematical than Zadeh.
The articles by Michael Wiesenberg on
www.cardplayer.com are very good as well.