To answer my own question… no, I’d say the DLC wasn’t worth the extra money, at least so far. The description said it was about gathering books of magic, etc, and I’d hoped that there would be some new gameplay involved, but nope. You just go to various locations and collect them. Some of the locations are dangerous, sure, but it’s all pretty meh.
Other than that, though, I’m liking it a lot. The atmosphere is great, the writing not bad, hunting up clues is a lot of fun. The game doesn’t hold your hand much, has you make deductions on your own for the most part. The glitchiness and bugs reported at launch aren’t in evidence anymore. The combat is still not terribly good, but the game doesn’t really revolve around it (but you can’t really avoid what is there).
It is definitely better than the other Call of Cthulhu game that came out last year. I played that one via Game Pass (or GwG? I can’t remember.) for a couple of hours and it was really mediocre.