Comment to the paragraph of the book, how to design database and any code patterns applyable?

Hello to the developers!

I've came into problem, while trying to develop a commentary module of the site. The main problem is the following: I have a task - to make an ability for users to make a comments to paragraph of the text, but I have no ideas, how to create a database structure for that.

Now we have four main entities in our system, which makes matter on this block: book, chapter (which in our case is called "part"), comment and user. So, user opens a book, on particular part, selects a word or a sentence in on a pages and writes a comment, or just read the comments, which are applied to the paragraph where this word or a sentence are contained.

Thanks a lot!