Gena Showalter 
Mad Zombie Party (The White Rabbit Chronicles Book 4) 

Ashton "Frosty" Martin is both elated and despondent when his dead girlfriend Kat comes back as a Witness just like Ali Bell’s sister Emma (who has helped Ali and friends throughout the series).Kat has a task for Frosty – help a fellow slayer and save him before it is too late. When Frosty gets to the place Kat directed him, he indeed finds a slayer – the disgraced Milla Marks, sister of neighbouring zombie crew leader River Marks – and the girl who betrayed the slayers to Anima Industries and caused Kat’s death.Milla is the last person on Earth Frosty wants to help or have anything at all to do with. But although she’s been banished from her home crew, Milla has learned that Anima has more tricks up their sleeves, and they’ve found a way to reverse or hinder Ali Bell’s new zombie-saving ability Anima is still out to find the cure for death and use the zombies to create an immortality serum. And Frosty, Milla, Ali, Cole and all their friends are about to be collateral damage again-unless they can find a way to work together and rebuild trust.It won’t be easy. But Frosty never liked doing anything the easy way.