It appears that this "hack" is just a function of how Google Book Search works. Google Book Search relies on books submitted in accordance with the Google Books Partner Program
, where publishers can control the page view restrictions
(20 to 100 percent), or turn off complete page views entirely and just allow short snippets.
It appears that Steve Rubel was basing his assumption of finding a hack or loophole in the system on O'Reilly's rather high percentage of how much of the book can be displayed before further page views are blocked.
Danny Sullivan at Search Engine Watch demystifies Steve's hack here
Steve Rubel thinks he's "hacked" Google Book Search, as he covers in his Read Most of O'Reilly's Hacks Books for Free Using Google post. In reality, I think he's just finding that Google Book Search operates exactly the way it is supposed to operate, to show you a percentage of a book that a publisher itself has allowed you to view online.
I think if O'Reilly perceived this as an issue, they'd adjust their page view restrictions accordingly.