Adding a book to a folder
From the Books page, find the book you wish to add to the folder and select the book menu options (three dots below the book). From there you will see the option 'Add to folder'
Select the folder from the dropdown and then 'Add to folder'
Moving a book to another folder
Click into the folder where you have added the book. Click on the 3 dots to open the book menu, and from here you can move a book to another folder in your account.
You can also choose a folder to save newly created books to. Just select the folder to add it to before you add any coverage!