The difference in quality between a (inflation adjusted) £99 book from 1962 and a today is depressingly bad. Print quality, that is, most old textbooks I've read are rubbish pedagogically.
Had a much better time with Tipler’s modern physics book later on.
I don't remember which Mir books I've read exactly, but I'm almost sure two of them were Irodov's Fundamental Laws of Mechanics and Saveliev's General Course. Maybe I was just too immature, but that thing was way too heavy for a standard high-schooler.