I admit this series is my favorite scenario set, along with the scenarios in H. P. Lovecraft's Dreamlands (Call of Cthulhu Horror Roleplaying), it is extremely good.
I could say it is complete, detailed, inventive, and expressive (swarming with adjectives? -- nope, just this review) -- but that doesn't tell you much.
Each scenario telegraphs itself well and is nicely contained.
Each scenario set is survivable, consistent, and cleanly laid out and set up. I've been playing and designing FRPGs since 1969 (gagh, with hex paper and chits), contributed to D&D (and edited the Expert Set) and was a playtester for Call of Cthulhu. Some things catch the soul of the game, have proper nuance of plot and could exist in the world as the game imagines it.
A good scenario collection is a joy to read as well as to play, fits the themes of both the game and the books the game is based on, and is consistent and intelligent. Once you finish with over a hundred and seventy pages in this bound collection (originally a boxed set, and realize it was done in early 1980s, the collection is amazing.
Even now it is five star good.
First, it offers two extra scenarios to start a group off with if you need to introduce players to either gaming or to Call of Cthulhu.
Next, it has a nicely constructed and well contained mainline of scenarios from the introduction to the hermetic evil order to the final confrontation.
It has a complete collection of handouts, music suggestions, and guidance for how to recover when things go astray (Call of Cthulhu allows that to happen).
One of the best things about it is that it is thorough and complete. Could it have gone more places and had a little less linear path -- sure, all you need is another hundred pages of text. Within the limits of the page length and the milieu, you couldn't ask for more.
Especially at the price this sells for.
Still my favorite after all these years.