A classic modern-supernatural game you can run from a single book.
I picked up Witchcraft (2nd ed) around 1999, and played with the book so often I had to get it rebound. While the supplements are useful (especially Mystery Codex), the core book could keep you playing for years on its own. The system is very easy to figure out, and has card and diceless variants included that I've used with success as well. The setting has the right amount of interesting groups to use in play, and you can re-skin them trivially to fit your own vision of an occult conspiracy setting.
And (at the time of this writing, and for quite some time) it's free.
This game features an interesting magic system, some great artwork, and a very usable dice engine. You will likely want to moderate how the experience system works (I found characters advancing way too quickly in some games), and also remember that guns are deadly in this game.
[5 of 5 Stars!]