I found out that I can write reviews in Google Book Search. I didn't know this feature existed, so maybe it's new.
I took the opportunity to add a "review" that informed that this particular volume is being proofread at Wikisource, http://books.google.com/books?id=EhRXuAD4aKAC
Is this something we should do systematically? There are 24 volumes in the Swedish Wikisource alone, that are based on scans from Google. (So I guess there could be hundreds in the English Wikisource.)
Did anybody try this already? Did you run into any problems?