Book printing and publishing is one the oldest industries in the world, which may lead you to wonder just how many books have existed in history. Obviously, we don't know the exact number, but we can use some math and statistics to make educated guesses.
In recent memory, the book publishing industry has been massive, raking in an average of about $25 billion per year and printing over 800 million paper-bound copies in the United States. Newer technologies like e-readers and audiobooks have transitioned publishing into the digital age and increased the volume of content that can be put out, using fewer raw materials.
However, the most important technological advancement for book publishing came in around 1440, in the form of the Gutenburg printing press. This allowed books to be mass-produced, rather than painstakingly written out by hand. The press accelerated book publishing in Europe from just a few thousand existing titles to millions, contributing to increased literacy.
Skip way ahead to 2010, and Google ran a project to count every book they could find on Earth, coming up with 129,864,880 unique titles in the process. That's a massive number, but it still doesn't account for the millions more which have surely been lost to history. Indexed books get rarer as the catalog reaches back in time. Google also mentioned that they intentionally excluded serialized pulp content and titles that only appeared in audio form.
Their data was based on titles that had received an International Standard Book Number (ISBN). The ISBN system was established in 1970 as a means for governments to identify book titles, so an unknown number of pre-1970 works have not been indexed. Self-published printed books are not required to be registered into the system unless the author plans to sell them on Amazon or through a bookstore. (Getting an ISBN for a self-published e-book is optional.) Generally, if the same title is available as an e-book, audiobook, paperback and a hardcover, it will have separate ISBNs for each format. Google said they tried to remove duplicate books from its count.
UNESCO estimates that 2.2 million new titles are added to the shelf globally each year. If we assume that 26.4 million new titles have been added in the last 12 years since Google's estimate of 129,864,880 in 2010, we can add those numbers together for an estimate of 156,264,880 titles in 2022. This does not include e-books, audiobooks nor books without an ISBN, which includes many self-published books. The actual number of books in the world would be much higher if we took those into consideration.