Hierarchy in Restful Url

Suppose there are resource like

1. user - users registered

2. book - books available in the library

3. favourite books - users favourite books.

What is the RESTful way to construct the URL for favourite books of a user?

I have confusion on hierarchy of resources in URL. How can I form correct URLs?

The following are the URLs that comes into my mind when there is request to find out favourite books of a user.

1. users/:user_id/favourites/books

2. users/:user_id/books/favourites

3. users/:user_id/favourite_books

What is the correct way of constructing the URLs?