Book #4 in the Mischief in Moonstone Series:

Three days before Halloween, Alyssa Swain finds a man dead in his car but, once she gets help, the body is gone. When scruffy, tall, dead man John Christopherson shows up alive on her doorstep, he insists she called him to help her get ready for a party in which dead relatives from decades and centuries ago plan to participate. Now the crazy man won’t leave her house or her heart. How can she keep him from crossing over into the afterlife at midnight on Halloween?