I completely agree about linking to this in docs! I added some reminder text about the 'printer-friendly version' link at the bottom of all 'book' pages to the end of the initial user's guide page, along with a link to (and credit given to metusela) the pre-formatted awesomeness linked in the initial post.

I know there are people working on extending the drupal core book module ( ie; http://drupal.org/project/e_book ), and I'm interested to see how they improve the actual printing output.