Winter’s Dawn by Arden Powell

Thomas Brighton, a professor of theoretical magic, has been accused of treason. Imprisoned in the bleak Blackwood Gaol as he awaits his trial, he is cut off from his magic and his studies: a fate worse than death for the scholar who has devoted his life to academia. His only company is Winter, the mysterious prisoner in the neighbouring cell. As Thomas’ trial drags nearer, their whispered conversations are the only thing keeping him from giving in to boredom and despair.
Winter is a radical, a murderer, and a traitor to the crown. Everything Thomas fears and looks down on. But as Blackwood continues to crush his spirit and his magic, Winter might be his only ally. And Thomas might be Winter’s only chance of escape. Because if Blackwood and its guards don’t kill them both, the hangman’s noose surely will.