As of November 2022
"Digital Rights Management", DRM, makes it so that you can't read a library ebook for longer than the time you're "borrowing" it, but that leads to other restrictions: To read a book like that, you need something that is compatible with the kind of DRM that was used. That means very specific e-readers, phone apps, web-viewers or desktop apps.
If you want to read the book on a different e-reader, or have your own favorite reading app that can't do DRM, you have two options:
Many libraries use services like Overdrive where the ebooks have the Adobe kind of DRM. There's also different versions of that, but this method here should get the books with the right kind, the removable kind.
When you download books with Adobe DRM from your library, you get to download an .acsm file. You log into your Adobe account. Your computer talks to the Adobe server and in the end, you have your book. Usually, you'd use something like the official Adobe Digital Editions program for this. This guide uses an alternative that gives you more options.
You can do this on Linux, Mac, Windows and probably other operating systems too.
The main tool used is Calibre. Calibre is a program for doing stuff with e-books. You can use it to organize them, to read them, to copy them to an e-reader, lots of other stuff. Even more functions can be added to it by installing plugins. We're going to use two plugins:
Install Calibre by following the instructions for your operating system.
Download the latest version of both plugins:
DeACSM
DeDRM
You want the zip with the name of the plugin, the topmost one.
Unzip the DeDRM zip. You're going to use the DeDRM_plugin.zip inside it and can ignore or delete the rest. Except maybe keep the ReadMe for the plugin. You probably won't need it for this here, but it has useful information for other stuff.
Open Calibre. On top you have a bunch of symbols, some of which may be hidden, requiring you to (in my version) click on a tiny area at the right border to view them. Find the one that says "Preferences". A new window opens. At the very bottom (you may have to scroll down), there's the Advanced section. There, click on the puzzle piece icon that says "Plugins". A new window opens.
In the plugins window, click "Load plugin from file" at the bottom right corner. Choose one of the zip files that you just downloaded. Acknowledge the security warning. When finished, repeat the same for the other zip file.
The plugin settings might open right after installation. You can also get them by searching for the plugin name in the plugin overview and double clicking the entry.
Open the DeACSM settings. DeACSM needs an Adobe ID to talk to the servers. You can either click "Create anonymous authorization" or log in with your actual Adobe account (although maybe not one that you would miss if it got suspended for using non-standard tools). Details here.
After you've done that and the plugin is authorized with Adobe, click "Export account encryption key" and remember where you save it to.
Open the DeDRM settings. Click "Adobe Digital Editions ebooks" and "Import Existing Keyfiles". Select the keyfile you just exportet from DeACSM.
That's it!
Borrow your ebook and click the option that lets you download it (as opposed to opening it in the browser). You should get an .acsm file. (If not, I'm sorry.)
Right-click the .acsm file and select "Open with Calibre" or whatever the option is called in your operating system.
It may take a while while DeACSM talks to the Adobe server and DeDRM strips the DRM.
After that, you have your ebook and can do with it whatever you want.
DeDRM works for a bunch of stuff, so if you ever buy an ebook and aren't happy with the ways you can read it, DeDRM can probably help with that, too.
If you use Linux and like the command line and don't want your books to end up in Calibre anyway (like I do… sadly, because Knock looks really cool), you might want to ignore this guide (oops, sorry if you followed it already, this note just didn't fit in at the beginning) and use Knock instead, which someone brought up after I posted this.