The older copies have paper that yellows, a likely indication of paper with a higher acid content than is advisable.
I do think the paper is better now, but time will tell.
There's a method by which you can fix your BoT paperbacks. If you can neatly get the cover off, set the book in a vise such that only about 1/4 to 3/8 inch of the spine is clear of the jaws of the vise.
Take a saw with a thin blade and down along the length of the spine make three or four dovetail shaped pairs of cuts into the spine about 1/8th inch deep. Then take some thread and wrap it tightly down in those cuts, in effect tying each pair of those dovetail cuts together with the thread and wrap the thread multiple times around. When finished wrapping the thread in the dovetail cuts, carefully glue the end of the thread down somewhere, preferably on the backside of the book and near its respective dovetail wrap.
Then glue the book cover back onto the spine, applying glue only on the spine edge and/or perhaps just a tiny bit along either face side and only about 1/8 inch in from the spine. Set the book aside for the glue to dry, with some other books on top of it for clamping weight.
If all of that sounds confusing, do a web search on how to fix "perfect bound" paperbacks. Perfect binding is the name for the glued binding used on most paperbacks and some hardbacks.
When I was at Westminster years ago, my wife had a small side business fixing copies of one of Van Til's books that was a required text. Problem was, students would buy the text and barely start reading before the pages would start flying out every time you opened the book. She used the method I've described above to fix those books, and I'll bet they still hold up to this day, decades later.