5It's a slow day at Scotland Yard, so Inspector Henry Tibbett takes a busman's holiday, immersing himself in the world of puzzling puzzlers. The hijinks kick off with an amusing gift: Someone unnamed has sent Henry the beginnings of a crossword puzzle. Even more mysterious: The clues point to the group of mystery writers to whom Henry has pledged to give a presentation. Most mysterious of all: None of the writers are who they claim to be, and one is a murderer. Which one' For that you'll need to solve the puzzle. Six across and then down, down, down.