The "About the Book" text can't be changed with a single snippet as it's part of the overview template. The will need to copy the 'templates/single-book-singlecolumn/overview.php' and 'templates/single-book-storefront/overview.php' files to a new folder in their theme called "mybooktable", to "<my theme>/mybooktable/single-book-singlecolumn/overview.php", to "<my theme>/mybooktable/single-book-storefront/overview.php", respectively. They can then change the About the Book text in those files. They can read more about this template system on the developer wiki here: https://github.com/authormedia/mybooktable/wiki/Template-System-Overview
If it's something that's commonly requested to override, I can add a filter in the next update to make it easier to change.