Cons: some situations are hard to believe given the circumstances
The body you are wearing used to be mine."
When Myfanwy Thomas wakes up in the rain, surrounded by bodies wearing latex gloves, she has no idea what her name is or how to pronounce it (it rhymes with Tiffany). The two letters in her coat pocket reveal both her identity and a choice: run or stay. A second attack convinces her that running away isn't an option so she decides to impersonate Thomas, a high ranking official in a secret British government organization (the Checquy) that deals with supernatural threats. Despite copious letters left by her 'predecessor' this is no easy task, made harder by the knowledge that one of her high ranking compatriots was behind the attacks on her and a traitor to the realm.
This is not The Bourne Identity for sf/urban fantasy fans. As a Rook, Myfanwy is in charge of the workings of the Checquy officers in Britain. She has meetings with various people and makes sure the realm is secure by covering things up and reporting them to the appropriate people. Her counterpart, Rook Gestalt, usually handles the field work side of things while she does the desk work. And she's very good at desk work. As the book progresses, the action picks up as Myfanwy is forced to attend to some of the field work, something her predecessor was ill suited for, but which the new Myfanwy is surprisingly adept at.
As a character Myfanwy is fascinating. She's learning about her former self while no longer being that person. She's more direct, more assertive and less willing to leave certain things to underlings. She's also more willing to use her own special abilities. You realize after a while that she's quite different from who she used to be, making it bizarre how few people comment on the change. It also makes for several ridiculous conversations where she's fishing for information she should already know. Sometimes this is commented on in the novel, a few times it is not.
The world-building is excellent. The author gives a lot of information via letters from Thomas, but they're written with dialogue and description, so the book never feels stilted. And while many of the letters are interspersed when specific information is needed, at times the letters are used to enhance the tension, by explaining a necessary side story while the main story builds up to an action sequence. The world of the Checquy is complex, with a school for children with special abilities, a complex hierarchy of the court and pawns, 'normals' who act as servants and compatriots but who can't rise to levels of power, an American office, etc. Learning about the world is almost as much fun as trying to figure out who the traitor is.
The author is aware of how ludicrous some of the powers and emergency situations are and often makes subtle jokes. When talking about Bath we learn,
According to Thomas the city had once been a veritable hotbed of manifestations, with every sorcerer, bunyip, golem, goblin, pict, pixie, demon, thylacine, gorgon, moron, cult, scum, mummy, rummy, groke, sphinx, minx, muse, flagellant, diva, reaver, weaver, reaper, scabbarder, scabmettler,... [the list continues for several lines] ogre, cat in shoes, dog in a hat, psychic and psychotic seemingly having decided that this was the hot spot to visit.The book is surprisingly fast paced given the partial narrative writing style. There's a fair amount of tension and enough action to keep things interesting.
If you like mysteries and intricate world-building, pick this up.