Koko by Peter Straub

This psychological horror novel centers around four Vietnam War veterans who reunite to confront a series of gruesome murders that eerily resemble war crimes they witnessed. The story delves into the complexities of trauma and the bonds of military brotherhood as the men embark on a harrowing journey to track down the enigmatic figure known only as "Koko," whose identity is as mysterious as the motives behind the killings. As they traverse the globe from the urban landscapes of the United States to the bustling streets of Southeast Asia, the veterans must grapple with their own inner demons and the possibility that the killer they seek may be one of their own. The 5277th greatest book of all time