The Mall of Cthulhu by Seamus Cooper

A decade ago, college student Laura Harker was saved from a fate worse than death at the hands (and fangs) of a centuries-old vampire priestess and her Satanic minions. Her rescuer, an awkward, geeky folklore student named Teddy, single-handedly slew the undead occupants of the Omega Alpha sorority house, spurred into heroic action by fate itself, inexorably intertwining his and Laura's destinies. After navigating her way through law school, Laura is now a junior FBI agent assigned to the Bureau's Boston office. Unfortunately, she finds her job involves more paperwork than adventure. When Ted stumbles onto a group of Cthulhu cultists planning to awaken the Old Ones through mystic incantations culled from the fabled Necronomicon, he and Laura must spring into action, traveling from Boston to the seemingly-peaceful suburbs of Providence and beyond, all the way to the sanity-shattering non-Euclidian alleyways and towers of dread R'lyeh itself, in order to prevent an innocent shopping center from turning into... The Mall of Cthulhu!