If you don't know about it yet, Wowio offers free book downloads- up to 3 a day from a selection of new and classic titles.  Some of the titles offered are not anything I would want so you do have to "wade" through the offerings to find what you like, but there are some great titles for schooling available. 

I just checked tonight and noticed several new non-fiction offerings that you might want to download for your own homeschool, so take a look if you have a chance.  

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