So I have two of the same book, Chimarvamidium, right? One is only worth 7gold, the other is 50 and the more expensive one is a skill book. Reading them, absolutely nothing is different about them. So... Why? Are there other skill books like this in the game, where some copies are skill books and other copies are cheap pieces of crap, or is this just a fluke?

this looks like a glitch

Likely a fluke.

Its just because it was a skill book. If I had a book that taught me a skill just by opening the cover I'd say it was more valuable than a regular version of the book.