The Curse of the Wendigo — Rick Yancey

While attempting to disprove that Homo vampiris, the vampire, could exist, Dr. Warthrop is asked by his former fiance to rescue her husband from the Wendigo, a creature that starves even as it gorges itself on human flesh, which has snatched him in the Canadian wilderness. Although Warthrop also considers the Wendigo to be fictitious, he relents and rescues her husband from death and starvation -- and then sees the man transform into a Wendigo.
Can the doctor and Will Henry hunt down the ultimate predator, who, like the legendary vampire, is neither living nor dead, whose hunger for human flesh is never satisfied? This second book in the Monstrumologist series explores the line between myth and reality, love and hate, genius and madness.