Those books would have been in the public domain even before 2000.
One of the good things about the 1976 copyright reform is that the copyright was simplified so that figuring out the copyright status merely requires you to know the death date of the author instead of trying to track down if various procedures were formally followed (I'd have preferred that it be a fixed term to make it even simpler, but it's still a simplification). To give you an idea of how complicated the older status can be, the copyright flowcharts that you see are basically a manifestation of "was this work copyrighted as of the copyright act taking effect?" (e.g., https://copyright.cornell.edu/publicdomain). Eventually, when the copyright clock ticks down on the intermediate period (and assuming no other copyright acts get passed to retroactively lengthen those periods), the determination will boil down to "death of author plus 70 years, or 95 years from publishing, or 120 years from creation."
"merely requires you to know the death date of the author"
That's a pretty big merely!
I can't tell you whether some obscure book is PD just by looking at the book. At worst you're basically working out when and if someone died just based on their name. Quick is Joe Blogs alive or dead?
Then there's the question of who the author is. I believe theres been a court case over Anne Franks diaries and whether her father was an author.
As I said, I'm not a fan of the "life plus N years" copyright term, but that's what the world has settled on (see Berne copyright convention--note that the life+50 is present at least as early as 1908, maybe as far back as 1886).
In the US, the law essentially requires that copyright owners need to explicitly inform the Copyright Office of the author's death date, or at least of evidence that the author was alive at a given point of time. Without that evidence, there is a complete defense that the 95/120 term is in effect. (See https://www.law.cornell.edu/uscode/text/17/302).
One of the good things about the 1976 copyright reform is that the copyright was simplified so that figuring out the copyright status merely requires you to know the death date of the author instead of trying to track down if various procedures were formally followed (I'd have preferred that it be a fixed term to make it even simpler, but it's still a simplification). To give you an idea of how complicated the older status can be, the copyright flowcharts that you see are basically a manifestation of "was this work copyrighted as of the copyright act taking effect?" (e.g., https://copyright.cornell.edu/publicdomain). Eventually, when the copyright clock ticks down on the intermediate period (and assuming no other copyright acts get passed to retroactively lengthen those periods), the determination will boil down to "death of author plus 70 years, or 95 years from publishing, or 120 years from creation."