OK, real fast: Unknown Armies (from Atlas Games) is a roleplaying game for playing “postmodern magick” and street-level sorcery: it’s very Tim Powers-y, and if you need me to explain who he is then I can only suggest that you go read at least On Stranger Tides, Last Call, and Declare RIGHT NOW and thank God later that you did. In the meantime, think ‘hedge magic in a modern urban setting, as practiced by people at high risk for OCD.’ It’s a fun game and a really fun game world.
Anyway… Chad Underkoffler (smart guy; good game designer) has written a FREE sourcebook for one of the groups mentioned in the game (The Order of St. Cecil: Vatican magick-busters with a distinct lack of body-hugging armor and vaguely blasphemous weaponry). It’s technically unofficial, except that it’s showing up on Atlas Games’ website: I assume that it’s officially unofficial so that they can determine whether there are people out there are still willing to pick up new material for the game. You can get a free download copy at Steve Jackson Game’s e23: you can also pick up a PDF of the core rules and supplements there, and NOT for free.
You should do all of that. If you game tabletop.