Awhile ago I found art instruction books that were written and illustrated by a great illustrator, Andrew Loomis. For some odd reason his books have been allowed to go out of print, so the only way to get them is to either pay some serious cash for them on ebay or if you're poor like me, download copies of them online.

They helped me and can be found here: [link]

There are other websites with scans from his books, too.