“We need enigmas,” Patrik Svensson, the Swedish journalist who authored this year’s most surprising sensation, has said. His book is full of them. For centuries, eels have baffled the world’s greatest minds. Aristotle pondered them. A teenage Freud dissected them for a month in a fruitless search for their testicles. No scientist has seen the creatures procreate or give birth. When he was younger, Svensson and his working-class father would fish for them at night—scenes he poetically recalls in more personal chapters that mystically echo the scientific history and explain his own fascination, which so many have since come to share.