Once Upon Stilettos by Shanna Swendson - 5 Stars

Katie, an executive assistant for Merlin, at Magic, Spells, and Illusions, is used to magic being used around her. As an immune, she helps spot trouble for her employer. But when Katie's immunity to magic disappears and it soon seems there is a spy in the mix at work, her world gets turned upside down. Katie sets out to find out who is up to no good in the company, all while keeping her own secret...that magic now affects her.