Which is the correct phrase to use? On the app or to the app

The lack of context is confusing the issue. Asking “What kind of book?” usually has the reply “detective, historical novel, horror, children’s, classic, reference, etc.”

With both examples, you probably not talking about “kind of book” but “the kind of file format in which the book appears”, i.e. you mean “In the case of books, which file formats can the app read?”

"What kind of a book can be added on the app?"= "What format of book can be added by use of or whilst you are using the app?" (added is

"What kind of a book can be added to the app?"= "What format of a book can be used as an addition/accessory to the app?" – this does not make a lot of sense.