Over the past couple of years, I have read not one, not two, but three books about magic libraries. I think that qualifies as a trend, so if you're toying with an idea about a magic library, now might be a good time to tackle it. Anyway, here they are.
1) The Library at Mount Char, by Scott Hawkins (2015)
A group of adopted siblings have been raised in the titular library, each focusing on a specific area of study. Now their father has gone missing, and they're locked out. With the power of the library up for grabs, the very nature of creation is at stake.
Of these comps, this one definitely has the least whimsy to it, almost at time venturing toward horror. Still, very well written and an interesting read.
Bonus, I randomly came across the agent who repped this: https://www.lizadawsonassociates.com/team/caitlin-blasdell/
2) The Library of the Unwritten, by A. J. Hackwith (2019, sequel scheduled for this year)
When authors don't finish their books, those books are collected in a library of Hell. Now there's one unwritten book wanted by multiple factions, including the angels of Heaven. Our main librarian must go out into the world to track it down and find out why it's so important.
A mix of light and dark, this might work if you're going for a vibe similar to the show Supernatural. Angels, demons, hellhounds, and gods from other pantheons.
3) The Invisible Library, by Genevieve Cogman (2016, with seventh book of the series scheduled for this year)
The Invisible Library is in a space between infinite alternate worlds, where Librarians go to collect unique books. Worlds can vary greatly in magic and tech, although the one focused on features Fae, vampires, and werewolves, along with some clockwork tech. What should be a routine book collection quickly turns into a sprawling investigation into murder, theft, and the Fae.
I only literally just finished the first book, so I can't say where the series goes, but it's definitely the most light-hearted of these three, a fun adventure romp that's a pretty quick read.