UPDATE (08 April 2013):
Just noticed that that distribution site for Loomis’s books that I originally linked to here is no longer in operation.
The good news, however, is that Loomis’s books have long been available in PDF format via Alex Hays’ Save Loomis! page.
AND the books are now being distributed by Illustration Age. See FREE Andrew Loomis Art Instruction Downloads. The writer on the Illustration Age site claims that Loomis’s books “are free to distribute because of their public domain status.”
AND they are available via the Internet Archive.