Updated 14.01.06
Art-Instruction Books of Andrew Loomis
My friend Tucho told me that he was using the books featured in this site to improve his technique, and actually I found them all very inspiring, not only for artists as talented as he is, but for everyone whith an interest in illustration. The page is also relevant since it´s hard to find printed copies of the books.
For more info on Loomis himself, you can read his biography here.
[Update] In a recent visit to the site this article points to, I found out that the books are no longer available there due to a letter of removal. On the other side, you can now find the books for sale in various online stores, like amazon.com