You can add a book to your favorites in two ways:
1. Firstly, navigate to your library.
2. Click the 3 dots next to the book you would like to add as a favorite.
3. Select 'Add to favorites'.
Additionally, when you finish a title you can scroll to the bottom of the 'Final summary' page and add it to your favourites from there by clicking on the heart next to 'Favorite it'.