The Ink Exchange by Melissa Marr, book 2 of the Faery Court series
The second book in Melissa Marr's Faery Court series is a winner. Leslie is a mortal friend of the Faery Summer Queen Aislinn. She know nothing of the paranormal life her friend lives until she gets a tatoo. A tatoo that changes her into the Dark King's Shadow Girl. A very, very good read.