Tree data structure

I’m working on a memorizing system

the program is designed for memorizing books
so what was the color of the dog in chapter one, what was is age how is mentioned in chapter two

what I mean is that the questions are relative to the position in the books, chapters, pages, etc

now every user has is won questions Sara has her one question on book A chapter B and Joe has his question in the same place.

So my question is how do I structure something like that

there is a big tree here
book collection inside the book collection there is a set of books documents
every document has is an array of books etc and then chapters and then pages
in addition, there are many questions from many users that will be in the books so it will really grow and be very hard to search

how do structure something like that?

Thanks in advance