The following steps are required to use ReadCube's Cloud Sync to backup your library (for reformatting or just peace of mind):

1.) Go into your preferences menu and under the "[update]" tab, make sure sync is enabled.

2.) Manually hit the sync button in your library and make sure it finishes.

3.) You can now reformat safely.  When you finish, just install ReadCube, once more enable syncing, and hit that sync button.  Your library will come back.