The Burning Page, by Genevieve Cogman
This is the third Invisible Library novel! Irene and Kai are on probation, so they're stuck with a lot of scutwork jobs, but then a straightforward return to the Library blows up in their faces. The Library is under attack by Alberich, who's got a scheme nobody has figured out yet -- nobody thought it was even possible to attack the Library at all.
I'm enjoying this series a lot, because the stories are not repetitive. Irene's situation keeps changing, depth is added, we find out more, and there's just a lot of well-developed story. Now there's this question about her parents, and her detective buddy Vale has this really interesting problem where he might become a Fae, and all sorts of interesting possible stories coming up.
A really pretty good YA fantasy series, lots of fun. I love these books.