Greetings. Nearly all of the books I've bought in the last several years have been in digital format. The excellent eyesight of my youth has declined, and now I usually need eyeglasses to read. I have a Kindle app on my phone, tablet, and laptop and can summon my entire digital library from any of those three devices. In the last week or so I've found several books online, by which I mean the entire text was available online to read, even though these books were then and remain for sale on Amazon.com and other online booksellers, some as paper books only but some additionally in digital format. Some were old and in the public domain, but others first published since 2000.
For example, I downloaded a .pdf version of George Edmundson's The Church in Rome in the First Century from the Christian Classics Ethereal Library (ccel.org) site. Then I dragged the file from my downloads folder and dropped it into Amazon's Send to Kindle page. After a few minutes the book was readable via the Kindle app on all three devices. The book consists of "EIGHT LECTURES PREACHED BEFORE THE UNIVERSITY OF OXFORD IN THE YEAR 1913" and ccel.org clearly identifies it in the public domain and that it "may be freely copied for non-commercial purposes as long as it is not modified." It's nice that Amazon allow this, especially since Amazon sells this work as a paperback and as a hardcover.
But there are others I've found online, downloadable for free, even though they were published in the last few decades and available in digital format. These were not particularly hard to find. I'm unsure why or even if the publishers allow for these books to be available for free to be read online or downloadable, and then even transmissible via Amazon's Send to Kindle page.