It's a handbook that's about helicopter flying and you should have one! Okay, fine... here's the official description.
"This is the FAA’s primary resource for helicopter operations knowledge; it is essential reading for applicants preparing for the exams for private, commercial, or flight instructor pilot certificates with a helicopter class rating. In addition to a basic introduction to the helicopter, readers will find chapters on aerodynamics, flight controls, systems, performance, ground procedures, basic and advanced flight maneuvers, emergencies and hazards, attitude instrument flying, night operations, and aeronautical decision-making.
Previously titled Rotorcraft Flying Handbook, this new edition is designed for use by flight instructors, pilots preparing for a helicopter rating FAA Knowledge and Practical exam, and helicopter-rated pilots wishing to improve their knowledge."