Update! Good news. It looks like the dropping of isbn.nu may be related to Google's inability to reach it during a crawl (although I can't see any sustained interruptions in our service in the last few months that affected isbn.nu's DNS or our network connection). Which makes sense as Brian's first part of his article didn't mention my techniques for being indexed. More news as it develops.
Another update: More good news. Google uncovered that their algorithms for stopping spammers seemed to have branded my site as such because of the massive numbers of redirects from my pages. Every book price results page has from 0 to 9 redirects (via an internal script) to the bookstores referenced. I should see a return of my content to Google in a few days, starting with the root pages.
My isbn.nu site has been branded, and not in the marketing sense. Google has dropped the 135,000 or so listings from my site from their index on Feb. 22, the day after Brian Livingston of Infoworld published part 1 of 2 articles about my site's indexing and commercial strategy (part 1, part 2).
What's ironic here, of course, is that I'm not a search engine spammer. The isbn.nu site uses a licensed data dump of over 2 million titles, which I transform into an index that can be traversed like a tree from page to page. Because I create static URLs that index to dynamic content, but each page is unique (i.e., the author page for Brian Livingston always lists the current set of books by Brian Livingston), I'm adhering to the highest principles of making a site accessible, traversible, and unique.
Not only have I discussed the site with Larry Page, who thought it was a great thing, but I've talked to several other Googlers who are well aware of me, and like the site and my approach. So I'm assuming it was an overzealous employee scanning the press for any mention of Google and lots of pages in the index. I'm going through a friendly channel at the moment to track down why I was dropped, but the concidence of Feb. 22 and the day after Livingston's article are way too high.