And I believe I fixed it.
It's slightly more complicated than what I'm going to outline below. But this description gives a reasonable overview, anyway:
- Google Books has a daily(?) quota for lookup request from a single IP (like from bookcrossing.com). - I haven't found out how much is allowed, but it must be many thousands (see below).
- Every day we did hit that quota sooner or later. That's when lookup started to fail, day by day.
- [skipping some tech stuff ... ;-> ]
- Every registered book has this "Global Overview" link. When you click that link you get sort of a - yeah - global overview. That page automatically calls a Google Books lookup.
-> "Spiders", those crawlers of search engines, follow rather any link they find. So these crawlers found these "Global Overview" pages.
They accounted for some two thousand Google Books lookups - per hour!
--> For the time being I've closed that "Global Overview" for anyone who's not logged in.
In the long run this probably doesn't make sense, I think this page should be avl for the public (what do you think?).
But as a short term solution this reliably locks out those overactive spiders. And in the background I've made a few more modifications to further leverage this issue.
(Yes, there are more regular ways to achieve this, but this is a quick, short term solution.)
---> As a result I expect Google Books Lookups to - more or less permanently - start working properly again within the next 24 hours.
On a side note this seems to indicate, well, I think I'm mostly back again.
But that's a different story to be told another time.