From the All Books view, find the book you wish to add to the folder and select the three green dots next to it. From there you will see the option 'Add to folder'

Select the folder from the dropdown and then 'Add to folder'

From the All Books view, click into the folder you want to move the book from. Click on the 3 dots to open the book menu, and from here you can move a book to another folder in your account.

Also, if you would like to import a book into another folder, simply click into the folder and then click on 'Import book':

Select your book from the drop down list and click add to folder.

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!