I have been a roleplayer for many years and a sizable portion of my games have been LARPs and I found Cthulhu Live 3rd Edition to be one of the best systems I have tried. This is not a tabletop system adapted to Live Action. It is designed to slow down the flow of the game so specific of Live games as little as possible enabling keepers and players to focus on the story at hand. The rules are simple enough to learn on the spot yet complete enough to solve every situation that might arise and versatile enough to be used in any setting including non cthulhu-specific ones.
Even the combat rules are designed to keep the rhythm active and moving forward, even in the most complicated combat scenes.
Rules aside, the book is also full of tips for keepers, suggestions and how-tos for props and costumes and a wealth of other ideas to help make your game all it can be. The book also contains a bestiary of those indescribable mythos creatures we love to fear.
A great read for any LARPer or Lovecraft fan.