A little-discussed but valuable part of Google Books is the About this book page. This is sort of like an enhanced card catalog view of the book — In addition to standard bibliographic data, it also has a variety of other useful information. For books with pictures an especially valuable part of this page is the Selected pages section, which has thumbnails for a selection of pictures in the book.
The About page is especially useful for full-view books, for which it has a wide variety of information, including popular passages, references from web pages & scholarly works, other editions, related books, & places mentioned (linked to Google Maps).
The About page is in strong contrast to the frontcover screen in full-view books, which is what’s linked from the main title entry for titles listed on the search results page. For full-view books Frontcover usually goes to the title-page of the book, which in most cases is much less useful than the About page.
In Limited-preview and Snippet-view books, the About page usually has much less information than in full-view books (This is apparently controlled by the publisher of the book). In these books, the Frontcover view really does go to the frontcover of the book, which, of course, is usually a colorful, exciting picture.
The URL for the frontcover screen then builds on this basic address …
… Which seems to imply that on some level the designers of Google Books see the About screen as the more basic, elemental unit for the book.