Magical Library where you can enter books

I read a book a while ago where I think the main character was a girl, and she goes to a library and can enter books and at some point she gets chased by monsters in the books. Pretty sure main side character is a boy and there's an old guy librarian. There were definitely sparkles at some point entering/exiting a book. Also, I think she wasn't supposed to enter the books? Please help!

This sounds like The Book That Wouldn't Burn by Mark Lawrence

Some overlap with The Night Librarian.

Story thieves?

Dragon's Green by Scarlett Thomas, maybe?

Going to throw The Forbidden Library out there, even though not everything matches up exactly.

The cat who saved books by Sosuke Natsukawa possibly?
Or maybe one of the books in the duology by Garth Nix where the first book is called The Left-handed booksellers of London and second one named The Sinister booksellers of Bath

I haven't read the book myself so I don't know whether there are sparkles or an old guy librarian, but Tilly and The Bookwanderers is the start of a juvenile series about a girl who can enter books.

Some book searches result in fascinating series of replies like this where I discover just how done-to-death a seemingly unusual idea really is!

Are you looking for The Invisible Library by Genevieve Cogman? or Maybe the Midnight Library by Matt Haig?

The Invisible Library by Genevieve Cogman?
The Eyre Affair by Jasper Fforde?