Ninja-the very word inspires awe and terror in equal measure. Master of espionage and assassination, stealth and concealment, the ninja's ability to move swiftly and silently gave rise to popular legends of amazing exploits, invincibility and supernatural powers.In ''Ninjutsu: The Art of Invisibility,'' Donn Draeger draws back the veil of mystery shrouding the arcane practices of feudal Japan's shadow warriors. Stripping away myth and exaggeration, Draeger reveals the secret tactics, exotic weapons, tricks and disguises that earned the ninja a reputation as history's most feared secret agents.