I just read your post and the good replies to it. Here are my thoughts on this as well.
It would help to know more about your system setup. If you run "inxi -Fxzd
" from the console terminal prompt, highlight the results, copy and paste them back here, that should provide enough information.
If these are free ebooks, or ebooks that you paid for, there is nothing wrong with using a DRM add-on to view them.
"Calibre" is an excellent ebook application that can be enhanced in many ways including adding various DRM plugins. There are already good posts in this forum on doing this just search for "Calibre". There are a few different DRM options including the good one you mentioned.
If you download the archive file from the link below and right-click it select extract here, there are various readme files to look at like "/home/user69/Downloads/DeDRM_tools-master/DeDRM_plugin_ReadMe.txt
apprenticeharper/DeDRM_tools: DeDRM tools for ebooks
"Just install the DeDRM plugin from the DeDRM_calibre_plugin folder, following the instructions and configuration directions provided in the ReadMe file and the help links in the plugin's configuration dialogs."
Restart Calibre after adding new plugins
Open calibre's Preferences dialog. Click on the "Plugins" button. Next, click on the button, "Load plugin from file". Navigate to the unzipped DeDRM_tools folder and, in the folder "DeDRM_calibre_plugin", find the file "DeDRM_plugin.zip". Click to select the file and select "Open". Click "Yes" in the "Are you sure?" dialog box. Click Apply. Click the "OK" button in the "Success" dialog box.
Coolreader Ebook Reader User-defined Settings and Linux Mint - Linux Mint Forums
- has very good links on Calibre and installing DRM options...
<Solved> Calibre Help Is No Help; I Give Up - Linux Mint Forums
Hope this helps ...
Animated Gif of adding this DRM Plugin to Calibre... Click image to view larger size, click the image again to make it larger or smaller.