AddALL couldn't find a digital version in any of its databases, so my guess is that there isn't a legal digital version.
I've poked around a bit and couldn't find a legal version, but lots of pirated ones.
One option that I use is to buy a used copy of the physical books and then download digital copies from the Internet. I consider it format shifting as I have legal copies of the physical books. As long as you have the physical copies it shouldn't be considered stealing.
My $0.02 worth.