Much of the plot is centered around photographs like the one on the cover. They are photographs Jacob's grandfather shows him to prove his stories, or that Jacob comes across in the process of finding out the truth. Riggs uses real vintage photos he has collected, and includes them with little or no re-touching. The images are all eerie in some way, and apart from a couple which seem shoe-horned in, complement the story by setting a tone and augmenting the text.
It's a very gripping book; we are drip-fed the truth and encouraged to figure the story out before Jacob does. It's a proper teen-fiction nail-biter and is a good read. It's very cinematic and the descriptions of people and place are vivid and raw. Well worth a read.