VBK

From MobileRead

VBK is an eBook format specially designed for text books. Readers are available for Windows and macOS X.

[edit] From their web site

What makes VitalBooks so powerful for learning? First, you get the exact, complete textbook for your course, not a substitute, not a truncated version, and not just a few ancillary web pages. When you purchase the VitalBook version of your text, you get a file that's fast, effecient, mobile, and interactive...just like you.

VitalBooks are unlike any e-book format you've seen before. The format was designed specifically for the teaching and learning environment, so VitalBooks...

Support large complex files — Other "picture-of-a-page" formats become unmanageable at 20-30 pages, but most textbooks are hundreds of pages long. VitalBooks are designed for efficient delivery, storage, search, and use of such large files (and, even better, large groups of these large files!).
Are portable — Many online books are tied to Web subscriptions available only from remote servers. A VitalBook is downloaded directly to the desktop. You own it, and it goes wherever your computer goes.
Allow rapid search across textbooks — Other reader programs may be able to search through one file at a time (providing a "find next..." or a list of hits with no context), but in a VitaBook, you can search by words or phrases across thousands of books — in just seconds. Search text, figures and tables, even your notes!
Have active learning tools — VitalBooks allow you to make and share notes and highlights right in your text, making it your own customized, dynamic version.
Enable linkages from other resources — You can easily insert hyperlinks to specific sections of VitalBooks from learning management systems, Microsoft Word™, PowerPoint™, .pdf or .html documents.
Carry digital rights management — Other formats deter copyright violation only by burdening the user. VitalBook files address file sharing, but don't limit the functions you want (like print, copy and paste, highlighting, etc.).