Hmm, there are some hints at some scenes in the novels, but I think they aren't really that bad. Of course something like this is always a bit spoilerish, but more in a "Hmm, that sounds interesting, I should read more" sort of way. Then again, I have read the RPG long after I started reading the novels, I might be totally wrong. Our World however definitely spoilers out the wazoo, since it describes every character and beasty Harry ever ran across. If you fear you might spoiler your players, just keep the books to yourself and give them an abbreviated version of the rules. There is a cheat sheet in the back of the book, that contains short rules and tables all on 2 pages, which can easily be copied on one piece of paper so they have the most important things at their disposal.
The RPG can be played in a lot of different styles, and as long as your group is into contemporary fantasy, you should be fine. If your players have never played an RPG before, you might want to start slow (feet in the water), and only allow pure mortals to get the ball rolling. You can introduce a number of powers and templates gradually over time, either as an enemy or as an ally. If they are experienced players, you can pretty much start with anything, the aspect rules need some getting used to, if you come from other games, but once you get into play, they start to come pretty natural, I think.