I originally purchased this supplement in hardcopy when it first came out way back in 1985. Unfortunately in the ensuing 25 years I managed to lose my copy (long story) to my everlasting regret.
This supplement contains a "comparative size chart" giving the Keeper and Investigators an excellent indication of the relative sizes between some typical CoC monsters and humans, a "scholarly" essay on the Ubiquity of Cthulhu, new creatures (both mundane and not), new Mythos races and deities, a Cthulhu Grimoire, a couple of other useful articles, and two scenarios, one of which is depicted in the cover illustration and is set in the Belgian Congo.
I stumbled across DriveThruRPG and discovered to my delight that they had PDF copies of many of the classic supplements and rules editions.
This particular PDF is extremely good quality. There was no difficulty whatsoever in reading any of the material, all the appropriate foldouts were faithfully reproduced with sufficient overlap that you should be able to print the pages and put them together if you desire. The only typos I discovered were extent in the original copy of the material as well, and all in all this was a superb effort on the part of DriveThru. The download was quick and easy, and the price is less than half what I paid for the original hard-copy back in the '80s.
All in all, worth every penny I spent on it. I strongly recommend it for the additional scenarios if nothing else, and it easily earns 5 stars from me.