I've found it also useful that when Google Books gives you a list of books, the note on each book listed shows a Google index phrase. These work as a link to a list of books on the same subject. I've found these links handy because they indicate how Google is grouping and indexing books but also as a means of speeding up a search for related books on the same subject or theme. However, as you point out, new books continue to appear and one can't always cound on Google indexing each new title the same way we each would if we were doing it - so the broader searches probably remain useful when you have the time and are just poking around to see what comes up.
Robert A. Mosher