Get the Audio Hijack app or any other app that lets you record computer audio. I’ve only tried it with Audio Hijack but I’m sure there must be others for Mac and PCs too. Open up the Libby app on your computer, which will let you listen to it on the computer. Start a new recording that captures system audio. Start the recorder (in this case, Audio Hijack’s System Audio capture) and then just turn on the Libby reader and let it run for the duration of the audiobook. I use an old computer so the process won’t interfere with my other work. I can turn the volume all the way down so it just records silently. The volume of the computer speakers is inconsequential, the recording of the MP3 that you’ll get is going to have a constant volume. Audio Hijack’s interface shows you an equalizer bar so that you can see that the sound is recording, and it shows you how much time has elapsed and how big the file is at any time. It looks like it uses about 40MB or so per hour of recording. When finished just go name the file and, if you wish, convert the mp3 to an m4b if that’s your preferred format using a program like Audiobook Binder. You’ll now have a file that you can keep, and you can “return” your audiobook early to the library so other people won’t have to needlessly wait.