While books are designed to keep songs that you don't use too often, you may want to remove those songs from your library but with an option to retrieve them at a later time. For instance, you may play songs around Christmas, but not have a need for those same songs through out the year. One way to do this is to save those songs to an OnSong Archive file to have the option of importing those songs in the future.
To do this, go to the Books Screen from within the Songs Menu by tapping on the "Books" button in the upper left corner. (In OnSong 2020, tap the Songs button > Songs tab on the bottom right > navigate to Books). Next, tap on the "Edit" button in the upper right corner of the screen to enter Editing Mode. This will place a red minus icon to the left of each book. Tap on the name of the book that you want to archive to open the Actions Menu. Choose to Export Songs. This will open the Export Screen with the songs in this book already selected.
Once in the Export screen, choose OnSong Archive as the file format to use for the export. To make things easier to restore later, ensure there is a checkmark next to the "As a single file" option in the Output Options Menu by tapping on the wrench icon in the lower right corner of the screen.
You'll want to decide where to store this file for future import. For instance, choose Dropbox to make sure that the file is stored off of your device. To do that, tap on the folder icon in the lower portion of the screen to open the Location Picker. When you're all done, tap the Export button in the upper right corner to begin the export and upload process.
Once you are certain that the file has been exported, you can then delete the book from the Books Screen either by swiping right to left over the book, or by tapping the Edit button to enter Editing Mode.
When you want to restore the files, create a new book in the Books Menu by tapping the + button to open the Add New Book Screen. Once created, select it to view the book in the Songs Menu. You can then import the file by tapping on the + button to Add Songs and import the file into the newly created book.