I accidentally deleted Buch. How can I get it back?
Undeleting accidentally deleted files isn’t specific for LibreOffice files.
It is only possible if the file physically still exists on the storage medium, and is only marked “deleted”. There is software to recover such files if not meanwhile overwritten with something else.
If you want to try recovery software, use your computer as little as possible till you are prepared to actually start the recovery attempt. Best chances if the file was on a mediim otherwise not much used.
If LibreOffice saves an edited document overwriting the old file from which it was loaded, it first creates a copy of the previous version with the extension .bak in the user’s LibO backup folder if the respective option is enabled.
The default backup folder is a subfolder of the user profile. The location depends on the OperatingSystem and is decribed here.
If you find a Buch.bak there, copy it to a working lokation and rename it back like Buch.odt. Then open it. Hopefully it isn’t too old. Did you often save intermediary?
If the option > Tools > Options > Load/Save > General > Always create a backup copy not was enabled, you need to try the above mentioned recovery.
I hadn’t the problem for years now, and can therefore not recommend specific software. But when I had to recover files years ago, I got the notion that the free Recuva, eg, was really good. PC-Inspector File Recovery also did its job well.