In this &;marvelously entertaining&; mystery, a hard-boiled Hollywood private eye investigates a murdered Munchkin on the set of The Wizard of Oz (Newsday).   A year after The Wizard of Oz&;s smash success, the yellow brick road is crumbling. The famous sets have been left standing on a soundstage in the depths of the MGM back lot in case the studio greenlights a sequel. But that doesn&;t explain what Judy Garland is doing there&;or why she finds a Munchkin in full costume, lying facedown with a knife buried in his back.   To avoid even a whiff of scandal and protect Judy&;s wholesome image, the studio boss hires Toby Peters, a Hollywood private detective with a reputation for discretion. But as Peters quickly learns, the real threat to Miss Garland isn&;t the tabloids&;it&;s the psychopathic killer who stalks the back lot and plans to kill the young actress next.   In addition to the murder mystery swirling around Judy Garland, the second Toby Peters novel features cameos from &;Clark Gable and Raymond Chandler [who] give an assist in this imaginative mystery recreated from yesterday&;s movie-land&; (The Plain Dealer, Cleveland).