This "bluing" of the back cover is ink. I'm not sure if was added at the Publisher level or Distribution level.
Most typically the ink runs along the edge. (but that's kind of hard to depict in a scan) In this example we have some ink overage that splashes well in to the back cover.
Luckily the front is unaffected.
This next book has an odd mark instead of a price, or at least it seems to be indicative of a price. Not being familiar with foreign concurrency, I don't understand the syntax of the "1/-" notation.
My best guess is that this is for either Canada or the European market at large.. maybe Australian..
This is a bit of an odd mark. Perhaps it was sent to someone in Military Service as part of a subscription or picked up at the corner drugstore from their base.
I am curious where such a book was intended to be (Geography wise) and can only guess how it left that county and made its way to the States.
Now THIS little gem seems specific to the UK.