The ebooks require the new version of the Sony Reader for Android app which Sony released just a couple weeks ago. They cannot be read on the Sony Reader iOS app, PC app, or on the Sony Reader (T1, T2, or earlier models).
Luckily most of the content can be found in other ebookstores, including the Kindle Store, Nook Kids, and elsewhere. I checked a half dozen titles and all were available elsewhere. While it is true that Sony is slightly ahead of the pack in adopting Epub3, they are also a couple steps behind in offering content.
The reason Sony baffles me is that I don't understand their ongoing investment in ebooks. Sure, I can see why they released a disappointing ereader last fall; hardware is cool, even a money- losing trophy like the Sony Reader. And I can understand why they are slowly expanding their ebookstore to more countries (most recently Germany), but I don't understand the Epub3 support.
Sony developed the new Android app in-house, and the same goes for the new ebookstore section. Developing the app is no small expense, and I don't see how they would recoup the cost by selling ebooks. Their ebook market share simply is not large enough; in fact it is widely estimated to be a fraction of a percent.
I am not saying that Sony's actions are right, wrong, smart, or foolish. I used to think Sony was a ship of fools, yes, but now that they have made multiple inexplicable announcements I don't know what is going on.
All I can say at this point is that Sony bears watching. They could be up to something interesting.