Call of Cthulhu is one of the classic roleplaying games that has stood the test of time. Published in 1981 it has gone through six editions and is now on its seventh. The rules are pretty much the same except it has gone to a percentile system but their books include a conversion system for older material. These roleplaying books allow for a more robust roleplaying experience than the starter set, reviewed by myself here. They are also a RPG fan favorite and their system has been in the top ten of RPGGeek for a very long time.
The Investigator book is geared towards players of the game: Introduction, character creation (including skills and occupations), organizations the player can be a part of, the life of an investigator, the main setting of the game in the 1920's, player advice, and references. Everything a player needs.
The Keeper rulebook has a more basic player creation section but it is geared towards running a game set in the Cthulhu mythos setting: the game system, combat, chases, sanity, magic, playing, tomes, grimoires, artifacts and devices, monsters/beasts/alien gods, two scenarios, and appendices. Very comprehensive and lots of good advice to make the game engaging and thrilling.
The Keeper Screen Pack has maps, reference booklet, screen, and two scenarios with handouts as well as blank character sheets. Very handy for running a good game.
I very much love this roleplaying game and it has improved a lot since I first played it in the early 1980's and of a much better quality. It could even be used for generic horror roleplaying without the Cthulhu Mythos universe but you would be limiting your possibilities if you do. Highly recommended.