Ghost in the Canteen (The Adventures of Lydia Trinket Book 1)

Homicidal toddlers, demon serial killers, handsome dead guys. Not to mention the PTA. Lydia Trinket has a few challenges, but she's got it all (mostly) under control. Until she goes to banish a poltergeist, and he banishes her instead.

Then Lydia is trapped in a netherworld with all the spirits she's sent there, and her long career as a ghost hunter looks a lot less successful from this side. She's surrounded by enemies in a constantly shifting landscape of dangers, where the whims of the dead outrank the laws of reality.

Survival would be good. Escape would be better. But Lydia is about to discover that there's no such thing as home free.