To add a book or an activity to your Favorites, simply tap on the heart icon on the top right corner from the preview page of a book/activity. You can also favorite a book from within the book as well. Once you click on the heart, it will automatically be saved to your Favorites.
You can view your Favorites directly on the bottom Navigation bar.
To remove a book from your Favorites, tap on the book to open it and then tap on the heart icon again.