Every time you close ReadCube, it keeps a backup of how it exists in that moment. When you next open the app and close it again, another backup is made. The third time the app is closed, the new backup which is created pushes the first one out; at any given time, ReadCube keeps backups from the last two times the application was closed.
If you need to restore ReadCube from one of those two backups, here is how to do so on a Mac.
Before you begin, create a separate backup of ReadCube as it currently exists and then fully close ReadCube by quitting it from your dock:
Then, you'll need to locate that backup file. To access it, open your Finder and type "command + shift + g"; this will open a search field:
Type into that search field what you see in the image above: ~/Library/preferences
This will take to to where the ReadCube backup lives within the preferences of your Mac. Search for a folder called "com.desktop.readcube":
Open that Folder and you'll see the "data" folder:
Now, inside that data folder, you'll see a lot of files but you want to look for a file that ends in .db, and two files that end with .db.backup.
That top file, the one that ends is just "db", is your current database file. The two underneath is that end in "db.backup" are your database backups.
What you want to do change one of those backup files to your current database file. To do this, rename your current database file (.db) with a different extension, like, .dboriginal or something like that. This is to make sure that ReadCube no longer sees it as your current database.
Then, look at one of your .backup files and remove the word "backup" from it, leaving .db as the extension. This will make that backup your current database.
Now, re-launch ReadCube and it will load with the last backup!