Agree. Nice extension. BTW the link to GitHub doesn't work in Jerboa.
Should also point out that, while you may want to delete the book when it expires, it will not self-destruct or delete itself because of the expiration date in the filename or mp3 tags. Once downloaded and saved in some folder or used in a third party player, unlike E.T., it can no longer "phone home" to Libby or Overdrive to get itself returned or deleted. It's lost its connection.
To add to the stew, I still recommended the Overdrive (not Libby) method, mostly because it can all be done on a phone (Android at least). You borrow the book (Libby App or Overdrive website. Then go to your library's "overdrive.com" website in a mobile browser. Under "loans" find your book and tap the button "Have Overdrive app Mac or Windows" (it also works in the mobile app), then tap the button that appears next "Download MP3". That will get you a small file with the extension .odm. That's a "license" file. Save it (remember where) and tap it. It will open the old Overdrive Android app to the Bookshelf and the files will start downloading. After they're downloaded use a file manager to go to where the files are stored. In internal memory that's: /storage/emulated/0/Android/data/com.overdrive.mobile.android.mediaconsole/files/OverDrive
If the files are stored on an external SD card, the path will be similar. You select where the files are stored in the "settings" of the Overdrive android app. Select "internal" or "external". Do not choose "app storage". That would store the books among the app's own files/code. They can't be accessed there. Also be sure the setting to auto-download books is turned on.
Then just copy the book folder with its files to wherever you want. But DO copy them out of that location somewhere else. Files in the Overdrive storage location can and do "phone home" and delete themselves on the expiration date!
Of course to make this work you do need the old defunct Overdrive android app. If you don't have it, you can get a copy of it online in many places. The APKPure website or the UPtoDown website are two that I know have it. It still works for downloading files using an ODM license file acquired on the website. It can still play the books. But it can no longer access any of your libraries. So you can no longer search for or borrow books through this app.
BTW this same process also works the same way on Mac or Windows - if you have, or can find, the old Overdrive Desktop App. But I like it on my phone because it is a self-contained process. No need for a computer and then transferring the end results to your phone.