This is book 1 of 8 in "The Invisible Library" series. 
The Library keeps the balance between Order (Dragons) and Chaos (Fae) throughout infinite worlds by sending Librarians to these worlds when certain books need to be lifted from the Alternate they show up in.

The main character, Irene, is a Librarian-spy. She has been tasked with training a desk jockey, Kai, while also being sent on a very dangerous mission to go to an alternate London and retrieve their copy of Grimm's Fairy Tales. There is an extra story in this copy that would be consequential if it were read by the inhabitants. While she's there, she notices that the balance has been tipped in favor of Chaos as the world has been infiltrated by Fae and someone has stolen the book. She has to get to know and trust her new assistant while also trying to find allies in this Alternate to help her figure out who stole the book, how to maintain a low profile, and get the book back to the Library. She meets a fae, Lord Silver, who tries to persuade her into his agenda using his seduction magic and a reputable detective, Mr. Vale, who teams up with her, but requires background information on her in order to trust her and her intentions, which could end very badly for her if that information got into the wrong hands. Oh yeah, and did I mention there are werewolves chasing them and death of a vampire? 😅