I first discovered Dover about a million years ago when I took my first calligraphy class. The teacher recommended that we buy and study their book Three Classics of Italian Calligraphy. (Being a total nerd, I followed this advice, and have not regretted it.)
Dover publishes oldies-but-goodies. They even have a reprint of the how-to-draw-animals book that was given to me by a neighbor who was an ACTUAL WORKING ARTIST (and therefore my hero) when I was growing up. The copy she gave me is falling apart--maybe I should replace it!
They also publish the most amazing coloring books!
And here's a really neat feature: Sign up for their "sampler" emails, and you get a weekly email with links to download pages from their books--coloring pages, mazes, puzzles. These were a staple in our household, and they make great inserts to send to sponsored kids!
Check them out! www.doverpublications.com