It's easy to call The Rook (by Daniel O'Malley) Harry Potter for grown-ups. It is thoroughly British, about a UK institution whose job is to hide magic from the outside world, and its concealed substrate of "unreality" is just as detailed and surprising (if more dangerous and adult). Hogwarts is a school for training wizards and the Ministry of Magic is its magical bureaucracy; its counterpart in The Rook is an estate for training run by the bureaucracy of The Chequy, but it's worth noting that the school (that one, at least) isn't even given a name. This is all about running the bureaucracy, which would be mundane if it didn't deal with a constant stream of supernatural threats and powers. For those undergoing Potter Withdrawal Symptoms (who are old enough to enjoy some of the details of running an organization), this is a fine substitute.
It's worth noting where the two are different. This is about patriotism where HP was about education -- but both are best when they capture the frantic, barely controlled chaos of a "busy day at work/school." In The Rook, an adult struggles to find out who she is, while in HP the theme is finding out who you are while growing up. The Rook has more sitations that are immediate life-and-death and a far higher body count -- but it's not "about death" in the way HP is (according to JK Rowling). Rather, it may be about contingency, choice, and the power of a fresh start even in the most challenging of circumstances. People are a lot more competent in this universe (and there's nothing nonsensical like the rules of Quidditch ... ) but it's also not as wonderful and eye-widening as HP.
As for the writing, it is wonderfully chaotic sometimes and there's a surprise a page. There's some odd shifts in tone from horror to humor and back again that come across as callous, so that may be part of why it's not as endearing as Rowling's characters. So many people die that it doesn't seem to matter when they do. That said, you may be able to argue that Myfawny Thomas (the central amnesiac thrust into the day-to-day operations of The Chequy) is more of a hero than Harry Potter himself. However, there's no Hermione or Ron around to care about, and I began to miss that as well.
Fundamentally I love books like this because they give me a plausible but convoluted world to explore and imagine. I don't want to mention too much of what goes on because the surprise is part of the fun. If this is a first effort from O'Malley, there definitely should be more.