I loved how imaginative these kids were, and how their changing relationships were explored. You could speculate that their quest was a product of their imaginations, but the history they discovered in East Liverpool would confirm Eleanor's story (that Poppy knew her name alone is proof), so I read this as a real ghost story. Creepy, but not too scary. I'd think the guy on the bus would (and should) scare some kids too. 
 As a real ghost story, then, I would've liked more at the end to feel more satisfied that all was well. Also, Zach's casual message about their theft felt like it should have been a more serious event with more explanation and possibly regret. 
 I listened to disc one on audio but couldn't get into it and switched to the print version.