This is an amazing series. We are using it as a foundation for my homeschooled son's high school biology. I have successfully downloaded all the units, although each one took between thirty minutes to an hour to download. I downloaded them onto our regular computer (windows), then transferred them onto an old ipad with no problems. You need ibooks on your ipad. For the second book, it said I downloaded it, but once I was in my own itunes library, I had to click on the book before the ipad transfer, and then the download went through. Hope that helps someone who is having trouble.

I'd love to rate it, but I can't get it to download!