It is always hazardous to try to guess the full rational of rejection.
So the best is to read back the guideline:
Your app should include features, content, and UI that elevate it beyond a repackaged website. If your app is not particularly useful, unique, or “app-like,” it doesn’t belong on the App Store. If your App doesn’t provide some sort of lasting entertainment value or adequate utility, it may not be accepted.
You only can say if it is really “app-like” and if it does "provide some sort of lasting entertainment value or adequate utility"
If I were to guess, I would suspect the second point. So you should try to bring some fun in your app.
So, just an idea to illustrate.
If the books are novels, you could list the names of main characters.
And create a quizz like : in which book does Sherlock appear ?
Hope that will help.