Interesting tidbit: why is there a key on the VitalBook icon? It turns out that before Bookshelf 3.7 the program was called "vitalsource KEY", hence the key in the icon. VitalSource used to sell DVDs with entire curriculums for distribution in universities, and they also preloaded what was called the "Portable Personal Library" on IBM computers. vitalsource KEY was what you used to view those books. Even farther back the program was called VitalViewer. What has stuck through all these years, however, is the name VitalBook. Heck, even the format's more or less the same (at least starting version 3; I couldn't find any earlier version for download).
Looking at the titles in the PPL, I can see why they don't offer it anymore. It's basically all public domain content. It's easy enough to get any of that off the Internet; who wants to pay for it?
Here is a snapshot of what VST was doing at the beginning of the 21st century.
Bonus: apparently VitalSource had a few TV spots. They are sort of weird.
https://www.youtube.com/user/wdmvst/videos