This book reads like a fascinating story, almost a biography, one of the most elusive and at the same time ubiquitous "numbers" in mathematics. History dates back to ancient Egypt, but European science has mastered this number relatively recently. Paul Nahin, a well-known popularizer of the exact sciences, weaves curious historical facts into the narrative, a discussion of mathematical problems and information about the applications of complex numbers and functions in such important problems as the laws of motion of the Kepler planets and AC electrical circuits. The publication is addressed to everyone who is interested in mathematics, including from a historical perspective.