You can get 25 Call of Cthulhu books for around $25 in this latest Humble Bundle—including the Starter Set, a deck of monster cards, and a colouring book

pcgamer.com:

Humble Bundle's dropped another great TTRPG deal—this time for Lovecraft lovers with its Call of Cthulhu book bundle. At the time of writing, the bundle has raised over $22,000 for the World Wildlife Fund. It's also a pretty damn good deal from a buyer's standpoint, too.

The bundle includes the game's Starter Set as well as a host of other supplements: including tips for the Keeper (the game's equivalent of a DM/GM), a PDF full of horrible monster cards to break your players with, campaigns like Alone Against the Tide, and supplements to help you run stories from occult pulp mysteries to modern horror.

My experience with the system's limited to a couple of one-shots, though I do recognise «Down Darker Trails»—a booklet that lets you transfer Call of Cthulhu to the weird wild west. While the system's a little tough to chew through (and perhaps a touch blase about its approach to sanity mechanics, a legacy that other systems like Critical Role's Candela Obscura are trying to veer away from) it's still a good time. 

If you'd rather not deal with a 100-sided die—usually just handled with two ten-sided dice in my experience, though our Keeper did have an actual 100-sided die that looked like the world's crappiest golfball—the bundle still gives you plenty of material that you can transfer to other systems. 

Our senior editor (and fellow TTRPG-liker) Robin Valentine highly recommends Trail of Cthulhu, which he says «combines a classic and exhaustively researched take on the Mythos setting with innovative mechanics designed to make running spooky mysteries a breeze.» Which is nice, seeing as Call of Cthulhu can sometimes make you crunch the numbers so much you get distracted from the horrors you're fighting. He also mentions