Ghosts of Weirdwood (Thieves of Weirdwood, 2) by Christian Mckay Heidicker

Fresh off of heroically saving their city from nightmarish monsters, 12-year-old reformed thieves Arthur and Wally are determined to join the Wardens of Weirdwood, defenders of the border between the Real and Imaginary worlds.

Their mission: defeat the Order of Eldar, a shadowy group that exploits the creatures of the Fae for their own gain. When the Order opens what seems to be a Rift between the worlds of the living and the dead, they set up a menagerie of ghosts to make money off those grieving for their lost loved ones.

As spirits begin to cross over into Kingsport, Arthur, Wally, and their ghost companion, Breeth, will have to return a fleet of dead souls to the other side of the Veil.