Sometimes a user can accidentally delete a KJots book. This is an inconvenience, but now the data can be recovered.
KJots automatically saves backup files of your books each time it is closed down. When you delete a book, the backup files remain. Note that old backup files will be removed 7 days after the original book is deleted.
KJots data files and backup files are stored in your local kde data directory. Usually this is
Open that directory in Dolphin.
Use the menu to open the find files dialog, and switch to the Content tab. Enter a phrase you know to be in the deleted book, and several backups of that book should be found. The name of the file is not relevant, but you can find the most up to date backup by looking at the last modified time of the files.
Simply copy the backup file to your home directory and open KJots. Use the file menu to import the backup book back into KJots.