Most of the classics and other public domain books that have survived the test of time have been digitized. Most modern books have too, since the majority of authors write on a computer. It's a relatively trivial thing to turn a digital manuscript into an ebook. Where the collection falls short, though, is in middle-aged books. Books that, while good, aren't really selling copies anymore. Books that were written on typewriters, and that require OCR software and extensive proofing to make ready. It's a little more effort for a little less reward, and so finding a book of that age tends to be a little tougher sometimes, in my experience.