Two-thirds into “The Great Unknown,” a mighty jam that searches out all the right tones, a truly soaring guitar solo is unleashed, signaling that the heavy metal pioneers still have fresh ways of channeling the abyss. In fact, in a blind listening test, this album could pass for a classic ‘80s-era Iron Maiden album; it has all the necessary ingredients, including apocalyptic riffs, imposing drum blasts, and, of course, Bruce Dickinson’s war-torn voice. The Book of Souls is a pensive tome that goes through numerous metamorphoses, increasing its smoldering energy with every step.