walter said:

Last time I checked there weren't any available through Gutenberg at the pretty basic level they are needed.

open-source textbooks are an idea whose time has come. there are now various entities gearing up to pursue them. i'd suggest that you take a good look around cyberspace. even if they aren't here yet -- and i don't think they are -- they will be soon, perhaps very soon. best of luck to you! -bowerbird