<p>Bit rot is a major problem with digital stuff. The e-books that I buy are generally science fiction - I read it once (slowly to savor every sentence) and may read it again later on but mostly it’s one-time enjoyment.</p>
<p>I have a dozen calculus textbooks at home - a lot of old stuff going back to the 1940s, some classics and some of the newer stuff from the kids’ courses. If someone has a problem, they can probably find a usable example in one or more of those books.</p>