This is something I'd picked up a while back, when it was on offer as an ebook, but hadn't got around to reading it till now - I thought it'd make a good counterpoint to the more 'different' fantasy I'm currently reading alongside it.
The basic idea of the book (which is apparently due to be the first in a series) is that of the Library, which is both a building and an organisation that connects various alternate worlds. That organisation employs Librarians, again with a capital letter, whose job it is to retrieve certain books from those alternate worlds and bring them back to the Library. In the case of this story, our protagonist is Irene (not her real name), a relatively-inexperienced Librarian who finds herself firstly saddled with an apprentice and secondly a mission that turns out to be vastly more complex than it had first appeared.
The alternate world she and her apprentice Kai end up in is much like our own in Victorian times, except with additional Fae involvement - vampires and werewolves are commonplace, as is the use of mechanical devices instead of honest horsepower. Irene and Kai end up finding an ally in that world's equivalent of Sherlock Holmes (with a dash of Lord Peter Wimsey in there too, as this Holmes is a sole detective, no Watson in sight) and a deadly enemy in a former-Librarian who is determined to get his hands on the book they seek, regardless of the human cost.
I found The Invisible Library enjoyable enough reading, though I think I was probably expecting something more than what I found - a workmanlike piece of writing, certainly, with clear effort expended on the worldbuilding but characters who didn't quite work for me. I did appreciate the effort taken by the author to avoid an obvious romance sub-plot, but the choice at the end of the book to plant Irene and Kai together in the world they have just been exploring seemed a little dull to me. Far better, in my opinion, to have them continuing to explore the possible alternate worlds out there than remain in just one, no matter how potentially interesting.