I agree with Brandy that Keys to Drawing is great. So glad she mentioned it! I've also used Hooked on Drawing, Drawing Is Basic (Jean Mormon Unsworth, quite inspiring), and Teaching Drawing from Art (Hurwitz & Wilson) as well as the books noted before. Hurwitz and Wilson discuss how much drawing in an overall program might be from observation, how much from remembered experience, how much from inspiration from masterworks, etc.
I never seem to settle on just one source, more's the pity; but for our Gifted and Talented 4th and 5th grade, Keys and Hooked were the main resources.