Using an algorithm that combined book information from multiple sources including libraries, WorldCat (OCLC) national union catalogs and commercial providers, Google estimated that there were "129,864,880" different books in the world. This number was, of course, constantly increasing.
Google's definition was inexact for various reasons including the detail that they "count hardcover and paperback books produced from the same text twice, but treat several pamphlets bound together by a library as a single book."
This information comes from Google's Inside Google Books blog, August 05, 2010. That provided other interesting tidbits such as:
"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."
"Our handling of serials is still imperfect. Serials cataloging practices vary widely across institutions. The volume descriptions are free-form and are often entered as an afterthought. For example, “volume 325, number 6”, “no. 325 sec. 6”, and “V325NO6” all describe the same bound volume. The same can be said for the vast holdings of the government documents in US libraries. At the moment we estimate that we know of 16 million bound serial and government document volumes. This number is likely to rise as our disambiguating algorithms become smarter.
"After we exclude serials, we can finally count all the books in the world. There are 129,864,880 of them. At least until Sunday."