Daniel O'Malley tackles the problem of establishing a secret world behind the real world by introducing his protagonist (Myfanwy Thomas, pronounced Miffany in the novel) after an even has burned away her memory.
Now for a brief tangent. Why when you lose the memory of who you are and what you do can you continue to recall part of things. I accept for the sake of the story that somethings remain, but some rhyme or reason would be nice. You can't recall your PIN, but the concept of a cash point (ATM) isn't an issue, neither is money. I'd be more intellectually accepting of a scenario where everything after the age of X is lost, or you keep a lot of basic information, but some things are stripped away (I know what a car is, but how do you use the steering wheel?) Anyway, as a plot device I think it's a hard sell, and far to often overlooks basic dissonances that can build up in the narrative.
Back to the main thread. Myfanwy has lost her memory, and more importantly for the plot, her entire sense of self. She finds a note in her pocket from her old self with some instructions and help. It appears that she knew that this was coming, and planned to make sure that new Myfanwy would have options. The first set is to get a new identity and go on the run or to stick around and find out what is going on. Like a sane person she chooses to run, until events intervene to prevent her from doing so. I appreciate the humanizing element that this creates, if I suddenly woke up in a field of corpses and didn't know why, I'd want to get away too.
Myfanwy ultimately can't run away, to the great benefit of the plot. So we get to learn who Myfanwy version 1 (dubbed "Thomas") was, what she did, and all the extended information that comes with it along with the protagonist. There is a magical world, there is a semi-governmental organization that handles the threats it faces (and they're mainly threats) and also recruits children in England who are born "special" and trains them to use their abilities. These special people become the pawns in the organization, from which the ruling council (Rooks, Chevaliers, Bishops, the Lord, the Lady) are chosen. Myfanwy is the titular Rook, who works maintaining domestic security in the UK.
I'll leave the plot alone from this point forward, if you read the book, the puzzles and the plot are worth discovering for yourself. The only real problem with the book is if the memory loss frame (and the associated prophetic foreshadowing) is a deal breaker. I was not convinced at the start that it would be handled adroitly enough to keep me in the novel. By the end I was happy to find that the treatment of new Myfanwy and old Thomas let me absorb large elements of world building, and make both characters more interesting without creating a massive disruption to the plot.
There's a sequel, we'll see how well it goes.