I think siggy only maintains a book review directory for reviews posted in ~BooksAndArticles, which is actually one of the big value-adds of posting there.

I also don't intend to collect reviews outside of siggy's directories. Finding all book reviews on SN is a much larger task than labeling reviews that have already been identified (the latter involves working with 100s of items, the former requires combing through million+ items)

That being said, these look like great reviews of great books, so I'll manually add them to the list for the next update.