Reply #8: "Plagiarism" isn't illegal, copyright infringement is

[A-Possum]

This is the first I've heard of this, but copyright does not cover concepts or ideas; it covers copying of the work itself.

That said, sometimes copyright holders make a claim that the overall work is so similar that it constitutes copyright infringement. The standards of Fair Use are:

“(1) the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes; (2) the nature of the copyrighted work; (3) the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and (4) the effect of the use upon the potential market for or value of the copyrighted work.”

So any copyright infringement case would take these into account. From what I know of both of these books, one being published as non-fiction and the other as fiction, and one not only not compromising the market for the other, but possibly increasing it, there is probably not much chance that an infringement claim will be successful. But as I said, I don't know the details of this particular suit and there's no link in the OP.

Keep in mind that writers make their living from copyright. Books do not create themselves; they are the result of hard work by the author. Under current laws, your work is copyrighted at creation. The internet has created new issues and pressures on the copyright law and also on authors, with new and competing demands. These will be pounded out as time goes along, as always.

Just because one particular suit may or may not have validity does not mean copyright law is "onerous." As to when a work becomes public domain, again, keep in mind that without the author creating it in the first place, it would not exist. The public didn't create it; though the public might benefit from it and after a certain number of years (related to either the lifetime of the author or the date of creation, depending on how it was copyrighted) it becomes public domain.