It seems reasonably within the bounds described by fair use, but nobody's ever tested that particular constellation of factors in a lawsuit, so there's no precedent - hand copying a book, that is.
17 U.S.C. § 107 is the fair use carveout.
Interestingly, digitizing and copying a book on your own, for your own private use, has also not been brought to court. Major rights holders seem to not want this particular fair use precedent to be established, which it likely would be, and might then invalidate crucial standing for other cases in which certain interpretations of fair use are preferred.
Digitally copying media you own is fair use. I'll die on that hill.
It doesn't grant commercial rights, you can't resell a copy as if it were the original, and so on, and so forth.
There's even a good case to be made that sharing a digitally copied work purchased legally, even to millions of people, 5 years after a book is first sold - for a vast majority of books, after 5 years, they've sold about 99.99% of the copies they're going to sell.
By sharing after the ~5 year mark, you're arguably doing marketing for the book, and if we cultivated a culture of direct donation to authors and content creators, it invalidates any of the reasons piracy is made illegal in the first place.
Right now publishers, studios, and platforms have a stranglehold on content markets, and the law serves them almost exclusively. It is exceedingly rare for the law to be invoked in defending or supporting an author or artist directly. It's very common for groups of wealthy lawyers LARPing as protectors of authors and artists to exploit the law and steal money from regular people.
Exclusively digital content should have a 3 year protected period, while physical works should get 5, whether it's text, audio, image, or video.
Once something is outside the protected period, it should be considered fair game for sharing until 20 years have passed, at which point it should enter public domain.
Copyright law serves two purposes - protecting and incentivizing content creators, and serving the interests of the public. Situations where a bunch of lawyers get rich by suing the pants off of regular people over technicalities is a despicable outcome.