Disclaimer. My spoiler tags are for Everwild as well as Everlost. I'm in the process of reading Everfound now so no spoilers for that book except my own speculations. I mention a wee bit of stuff on UnWind as well because I gush how great Shusterman is.
Everlost is about the in-between place where dead children end up if they got lost on their way to "getting to where it is they are going".
Nick and Allie die in a car crash and after 9 months they wake up in a forest. Everlost is built of the dead spots where people died and places of importance that are no longer here UnWind was kids vs. adults/government but the plot synapses promises to extend to other powerless people in a totalitarian government. I'm reading the Skinjackers trilogy while I'm waiting for the next book.
Everlost is childish in some ways but it really fits and made it creepier in a way. It made sense the love connections these kids made were chaste. Kids develop crushes without attaching passion to it after all. Nick has an unfortunate crush on Mary which lands him into hot water with Vari. Vari is a musicial genuis and was nine years old when he died. He has curly blonde hair and exists to serve Mary. Nick doesn't realise these kids may appear to be kids forever but have old souls. If the Everlost kids didn't have their own form of bloodless lust the implications here would be pretty icky. This changes in the Milos storyline of Everwild. Milos died at 17 and falls in love with every girl he meets but he can possess living bodies so his feelings are more sexual than say Nick's for Mary. In Everlost they had theories why most kids don't die at 17 and his reason is explained in Everwild but not why more older teenagers aren't there.
I just started Everfound and the major theme seems to be not wanting to be left alone so Everlost started out the nature of these relationships perfectly.