The Sarah Milton Chronicles — J.B. Cameron

While investigating the brutal murders committed by a mysterious serial killer known only as "Raithe," bookish LAPD Detective Sarah Milton is unprepared to have her entire world turned upside down. Innate powers to see the dead, lying dormant since her mother's murder, have reawakened in her after a near-fatal shooting. Along for the ride is Sarah's irrepressible thirteen-year-old childhood "imaginary friend," Anna Nigma, a most atypical poltergeist. Amid fears for her sanity, Sarah must come to grips with the realization that her reality is now a mix of the natural and supernatural, where powerful, ancient mystic symbols can grant amazing powers over life and death, and paranormal influence extends even into her current murder investigation. Forced to hide her abilities from everyone, Sarah, aided by her spectral friend, has no choice but to bring Raithe to justice on her own, before the sinister forces behind his murder spree claim yet another victim.