My book, if and when it is accepted, will be in the nonfiction category, which has 1521 books listed as of the moment. No doubt the number will rise quickly. I don't believe that many people will scroll through thousands of books looking for one that catches their eye. People will have to find our books some other way. Michi makes some good suggestions.
This contrasts mightily with the way Amazon does it, especially the Kindle Singles. I have had two published that way. You submit a proposal and Amazon agrees to publish or not. If they do, and if you are lucky, they will give it a prominent place on their site, even showing its cover in one of the categories. I found that the number of copies I sold was closely related to how prominentely Amazon placed the book. Of course these are not "enhanced" books and contain only a few charts and photos.
If I go to the Kindle site today and search for the topic of one of my books, "extreme weather," the book is near the top. It costs 99 cents and is sandwiched between two books that cost $36 and $91, respectively. What a bargain!
I also have a connection to a couple of websites that gave each book a review and I could tell that had an immediate effect.
But as soon as neither Amazon nor any website was featuring the books, sales went right down.
Couldn't Apple use our keywords to make our books findable in a Google search?
Jim.