I originally purchased this supplement in hardcopy when it was originally published in 1983. Subsequently, I managed to lose it, and was delighted to discover that DriveThruRPG had it for sale again in PDF format.
This supplement contains two "scholarly" essays that are well enough written that I used them as player handouts in my original game when my player group was researching the nature of the "threat" during some "downtime" between adventures! In addition there are four new scenarios, one of which, "The Secret of Castronegro," was a particular hit with my group and became the stuff of legends as time went by. Another of the scenarios, "Paper Chase," is an excellent introductory scenario, though it needs tweaking if you want it to be something more in keeping with the mythos experience (as written, it serves to confuse beginning players more than anything else). There are useful articles on Prisons, insanity and skills introducing new types of each, and a section of new Mythos deities, races and monsters.
Overall, the quality of this PDF is exceptional -- there were no parts where any of it was hard to read, the reproduction is high quality and even the "Sanity Quiz" (which appears on a dark background) is highly legible. Download time was extremely quick, and I can safely say I was delighted to have a copy of this to add to my slowly growing library of replacements for missing supplements. All in all, I have to give this one a five, not only because of the quality of the material, but also due to the quality of the reproduction. Even better, I paid just over half as much for this as I did for the original supplement when it was published in 1983!