[usef-]

Is that easy to tell for each book, though?

I usually hear that it's extremely hard to track down the copyright owners for many older things. That's the reason the film/video industry often gives for not digitising the backlog online, and I would expect their volume to be much easier to handle than the book industry.

[golem14]

It’s not easy, but there are good heuristics. If it fetches 10s of thousands at an auction, like an original Gutenberg bible or something, don’t destroy. If it’s my thesis from high school, destroying while making a copy is ok.

It’s really not important to come with a totally bulletproof 100% accurate rule here.