If you're like me, you already have most of the books. But if not, take a look at Bundle of Holding (http://www.bundleofholding.com). It's just renewed its two Ars Magica 5 offers. This is a great way to get a bunch of books cheaply and help a charity in doing so.
Just bagged the extended version - great!
Sure I've got the books, but I don't have electronic versions.
If they did a Realms of Power bundle (or probably any other 5th ed. bundle) I'd purchase that too.
Thanks. I was missing some of these PDFs. No more.