So after all is said and done, how many clusters does our algorithm come up with? The answer changes every time the computation is performed, as we accumulate more data and fine-tune the algorithm. The current number is around 210 million.
Is that a final number of books in the world? Not quite. We still have to exclude non-books such as microforms (8 million), audio recordings (4.5 million), videos (2 million), maps (another 2 million), t-shirts with ISBNs (about one thousand), turkey probes (1, added to a library catalog as an April Fools joke), and other items for which we receive catalog entries.
Counting only things that are printed and bound, we arrive at about 146 million. This is our best answer today. It will change as we get more data and become more adept at interpreting what we already have.