A game worth reading, even if you don't plan to use it immediately: it contains a handful of concrete advices on how to run "cosmic horror" games, how to create investigations (this chapters can be a great addition for Call of Cthulhu and other game Keepers), and the icing on the cake: a new take on the old Lovecraftian "pantheon" and the Realms from beyond the walls of sleep.
This game mechanics are simple but effective: Say what your character wants to do. The GM define what happen if the character fails his roll. Then roll 2D6, add the relevant attributes: if you beat 8, you get what you want, otherwise…
Is it perfect? Not if you want an extensive game background and tons of Skills/professions/weapons. But if you're searching for a simple game, easy to learn and yet full of depth and neat ideas, take a look at Rats in the Walls, you won’t regret it!