Books can be just text. They can be text and images. Since the rise of the ebook, books have begun adding video and sound in certain ways (most often in textbooks.) Of course, that's not all a book can be and now Google has set up a store specifically for books "that cannot be printed", which appears to mean experimental pieces rather than something like a textbook with audio and video bits added in. The books are designed to be read on a smartphone, though Google warns that you try before you buy in case the book doesn't play well with your device.
There are only two books available at launch and both will regularly sell for $4.25, but will only cost $3.25 as part of an opening deal. One is "The Truth About Cats & Dogs", which is described as a failed collaboration between novelist Joe Dunthorne and poet Sam Riviere, and it plays out as the journal entries between the two where you can switch between at will. The other, "Entrances & Exits", is a bit more interesting because it plays out almost like a point-and-click adventure where you follow the story through Google Street View.
Beyond those, there are two more books expected in "Spring 2016" as well as a number of ideas for potential future books. One story idea is for a book that "grows biologically" (which is far more complicated an idea than it sounds), and another that will be a "story that tells itself", meaning that it will start with one story then use something within that story to generate another.
Overall, it's an ambitious and interesting idea. It appears to be presented well and definitely has the possibility of being popular, but Google will have to put more marketing muscle behind it for that to happen. We really do hope that this isn't an experiment that Google releases then forgets.
source: Editions at Play