Uff da, that was one of the best debuts I've read in a while. It all feels very familiar in the beginning: The nightjars (a kind of bird more commonly known as nighthawks in the US) act as soul-bodies; this feels like Phillip Pullman's daemons (or Lauren Beukes' familiars, but I don't think as many people have read Zoo City.) Much of the action takes place in an alternate London known as the Rookery; there are so many alternate Londons that this listicle of ten has a half dozen comments pointing out several not on the list. The overt plot opens much like Neverwhere, with an office drone getting sucked out of her routine and submerged into a magic world (though The Nightjar's protagonist doesn't suck like Richard Meyhew.) I totally thought I knew where this all was going, but whoo baby does Hewitt deliver a series of brutal shocks in the end.
This is the interesting part: sometimes when writers deliver a twist, it's unearned in the narrative. Either they've been withholding information, or they're just being perverse, or they weren't good enough setting it up. (See, for example: Dany's burning of King's Landing, which was at least the latter two, if not all three.) Not so, Hewitt. She gives you all the information, but in such a way that the reader jumps to several erroneous conclusions. So when she puts you right in the end, it's no one's fault but your own for making an ass out of u and me. It's very, very well done.