I believe that it is difficult, if not impossible to find this out. To find the sum of human publishing isn't easy. Then to find out whether digital versions are available takes a bit of time.
It would involve taking the catalogues of say, the British Library and the Library of Congress, removing duplicate books (e.g. different editions of Jane Austen) and then searching for ebooks of their catalogue entries.
This task would have to be repeated for all languages, and on a daily basis, as new books are published or old ones rediscovered.
And you haven't said whether you are restricting yourself to legal books or whether you would count all the stuff on torrent sites.
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