The Left-Handed Booksellers of London by Garth Nix

Susan Arkshaw is excited to be turning 18.  She plans to leave home and her slightly spacy mum and go to London to find her dad before she starts college. When she gets to London she visits a man whom she thinks might be her father, or at least might know something about him.  When he ends up dead, she finds herself swept up in a world she never knew existed. A left-handed bookseller (i.e. guardian of the interface between humans and the fairy realm), named Merlin, saves her from demons and other nasty supernatural creatures who seem to have taken an interest in her. Merlin and his right-handed sister, Vivien, work together to find out who Susan really is and why a particularly nasty part of the fey realm is trying to kidnap her. 

I don't think there has been a Garth Nix book that I didn't like.  This one isn't his best, but it is still very fun.  Early in the book the Merlin character talks about how children's fantasy authors cause problems for "booksellers" because they accidently unearth ancient rituals and stories relating to the fey and include them in the books. Then, through the rest of the story, Nix makes very subtle references to creatures and things in children's fantasy stories.  It is as if Nix said to himself, "what would a world be like if all the magic in classic children's fantasy were real," and then wrote a book based on that. Nix is nothing if not clever. Susan is a sympathetic character, and Merlin is just fun. He is a enthusiastic cross dresser and Nix has a great time dressing him up in outrageous outfits throughout the book.  The second in the series just came out last month and I will probably end up reading it. (2020, 416 p)