Nox Winters and the midnight wolf

"For twelve-year-old Nox Winters, the town of Evergreen isn't home -- it's a tourist trap in middle-of-nowhere Maine for amateur cryptozoologists hoping to spot its infamous legend, a batlike creature known as the Evergreen Devil. But his twin brother, Noah, has been wasting away from an illness that has left medical professionals stumped, and the doctors here in Evergreen are his last hope. And Nox goes where Noah goes. But as Noah's condition worsens, Nox begins to suspect that Evergreen is doing more harm than good -- and that there might be some truth to its tales of fearsome cryptids and mysterious disappearances. When Noah falls into a deep sleep that Nox can't wake him from, Nox's only chance at saving him lies in the surrounding forest; there, he discovers a hidden world where it's always nighttime, full of powerful magic, supernatural creatures, and capricious minor gods. Nox must venture deeper into an endless night and successfully bargain with the terrifying Keepers of the forest for a cure to Noah's ailment or else neither twin will leave Evergreen alive."--