The Google Books blog has an interesting post on their current rough count of, using a definition of “book” that is “very close to what ISBNs are supposed to represent” if only all books had ISBNs and all non-books didn’t. So they do “count” different versions of the same work: lots of different editions of Hamlet, with different forewords, commentaries, and translated into different languages, as well as hardcovers and paperbacks of the same editions.
On that basis, the current tabulation is 129,864,880 books. That number is likely to decline, however, as their algorithms get better at detecting libraries’ bound collections of serials and government documents.
More interesting to me than the number itself is the description of what else they’ve found along the way: about 1,000 t-shirts with ISBNs; 4.5 million audio recordings; and all manner of cataloging and metadata errors. “We’ve seen anywhere from two to 1,500 books assigned the same ISBN.”