The Invisible Library by Genevieve Cogman, 341 pages.
I read this book during vacation on the recommendation of Kara, who wrote a review of it here, and I quite enjoyed it.
It starts out slow as the author world-builds in that sort of "well you know how it is with dragons" way, where she is letting you know that there will be dragons, magic, steampunk, and fairies sprinkled throughout the "alternates" we'll encounter, without letting you know exactly what the rules are. It seems a little coy and contrived at first, and maybe gives the story a slower pace at the beginning, but the convoluted story does sort itself out and the author's well-drawn characters deliver a very good story.
Irene, a newish librarian, is sent out into one of the many alternate worlds to find a unique copy of
Grimm's Fairytales. Being given this task at this time strikes Irene as a little strange since she has just returned from a mission and is due some downtime. Stranger still, she is assigned a student with some secrets and this is increasingly looking like a dangerous mission. When the many complications, from decapitated vampires, fairy ambassadors, private detectives, to renegade librarians arise, Irene has to be both resourceful and lucky to see the mission to its end. Sticking with the book through the unwieldy beginning is worth it.