This is book 4 in the series of October Daye novels, and as a lot of the plot points in the book will make little sense without reading the previous ones, I would recommend starting with book one, "Rosemary and Rue".
It's no secret that the Queen of the Mists hates Sir October "Toby" Daye, so it's rather a surprise to Toby and most of her friends when the faerie queen summons Toby and bestows the title and property of the Countess of Goldengreen upon her. Pretty sure it's some sort of elaborate trap, Toby doesn't really have time to ponder it, as someone seems determined to kill some of her nearest and dearest. Lily, the Lady of the Tea Gardens is dying, and shortly after, Luna, the wife of Toby's liege lord is also struck down by a mysterious poison. Then Tybalt reveals that several of the cats in his kingdom are dying. Toby is convinced it is the work of Oleander de Merelands (an evil faerie who helped conspire to turn Toby into a fish and stole 14 years of her life), but no one else believes her.
Rayselline Torquill, the insane daughter of Luna and Sylvester accuses Toby of the attacks, and the Queen of the Mists is only too happy to have Toby arrested. Luckily, Toby has friends who are willing to do pretty much anything for her, including risking their lives to break her out of faerie jail. Toby needs to prove who is poisoning her friends, try to find a way to save them and clear her own name.
While I've enjoyed the previous three October Daye books, and especially admired the world building, this is the first one I actually had trouble putting down. I kept thinking about it when I wasn't reading, and wanting to find out what happened next. While Toby takes a while to grow on you as a heroine, by this book, you're firmly on her side, and cheering on Connor, Tybalt and Quentin as they loyally refuse to abandon her. The surprising reappearance of Amadine, Toby's mother, and the discoveries Toby made about her identity also intrigued me, and explains quite a bit about events in previous books. The world building continues to be excellent, and I love how McGuire manages to combine modern life with cars, computers and mobile phones with magic, glamours, and a wealth of faerie mythology. The series has really got its hooks into me now, and I can't wait to see what will happen next.