TheLibrarian

If you're planning on running a published scenario (I'd recommend Amidst the Ancient Trees from the core rulebook, the Haunting from the quickstart rules, or Crack'd and Crook'd Manse from Mansions of Madness) then read it, reread it, make some notes, and reread it again. Having a solid knowledge of the location and nature of the clues and important locations will make it much easier to run the adventure and, let's face it, adapt to the completely off the rails choices any group of players, particularly new ones, are bound to make. If you're running something original, then it can't hurt to read a mystery or detective story or two. A mystery story is paced differently from an adventure and refreshing yourself on the tropes is always good, in my experience. Your group should be delighted to have such a dedicated GM!