What was Dame Agatha really like and why have readers bought her more than anybody but Shakespeare? Those are the current Agatha Christie mysteries, and Murdoch's chatty, affectionate Part I (""Writer"") and Part II (""Writings"") leave the puzzles thoroughly unsolved. Small wonder. The Christie executors, continuing in AC's tip-toeing footsteps, decline to cooperate with would-be Boswells and even refuse permission to quote from the canon. Murdoch compounds the handicap by swearing himself to good-sport secrecy: no murderers unmasked, except Roger Ackroyd's. Nonetheless, the miniature biography (filled out with reasonable enough servings of speculation) does succeed in locating the locked doors--first husband trouble, an amnesiac episode, the privacy fetish--even if it fails to open them. And Murdoch's critical essays, though they glide primly along the surface, astutely salute Christie's forbears (Poe, Collins, Doyle, Chesterton) and competitors from Edgar Wallace to Patricia Moyes. The judgments are cozy and debatable (Death Comes as the End a masterpiece?), and the reward for reaching the last page of narrative is a bouquet of appendices sure to entrance the Christie-for-Christmas crowd. Innumerable cuts above Ramsey's Agatha Christie: Mistress of Mystery (1967), but the good Dame, as ever, is holding onto her secrets until the last possible moment.