The "About this material" page can be found in the left-hand corner of the main table of contents:
Note that the button will not appear while on the "Reporting" tab.
The button can also be found in the side table of contents while in a section:
Clicking the "About this material" button will load a modal on the page:
This modal contains information like the zyBook ISBN, author information, release date, and summary. If your zyBook contains more than one subject, you can check this information by changing the tabs for which subject you would like to look at (in the example above, the zyBook has 3 different zyBooks: Computing Technology for All, Programming in C++, and Troubleshooting Basics).