The Invisible Library was utterly delightful, with huge servings of awesome-sauce on the side. It gave me the happies on almost every page. I mean, there's a Library, so, yeah. And dragons. You'd think that would be enough (that would be enough for me). But, no, there's more! There's a super-smart detective who could convincingly be played by Benedict Cumberbatch. And airships. And remotely-controlled alligators, because, every plot can be improved by the addition of remote-control alligators.
(And Cogman gets the tone pitch perfect: just self-aware enough to take itself seriously without being ridiculous.)
Irene is a fabulous character, right up there with Prunella (from Sorcerer to the Crown. This book is right up there with Sorcerer to the Crown. Possibly even surpasses it. Wouldn't want my life to depend on picking one over the other.) She's competent, firm, thinks on her feet, rises to the occasion, but she's also still a junior Librarian who doesn't have all the information or experience she needs. She has moments of panic, doubt and sheer frustration and it's lovely to watch her deal with them—actually, it's lovely to listen to her narrate how she deals with them.
It gets better. There are, not one, but two really hot guys who spend the whole book being impressed by Irene, talking to her as equals and respecting her opinions and decisions. I could eat this stuff with a spoon; it's better than ice cream. There is a wonderfully complex rivalry between Irene and another woman Librarian. There's a fascinating alternate London, plausibly steampunk and infested with chaos (in the form of Fae, vampires and werewolves, among other things). And there's the Library, with its strange rules, twisted politics and mysterious purpose.
It's all fun as heck, and I can't wait to dive into the next book!
This might not technically be YA, since the characters are over twenty, but it would work just as well for YA or adult.
I'm feeling another music analogy this time: "Starlight" by Muse.