Gr 2-4-- In this sequel to The Mysterious Cases of Mr. Pin (Atheneum, 1989), the rock hopper penguin detective with a passion for chocolate returns to tackle two new cases. Both feature prominent Chicago landmarks. In the first story, set at the Civic Opera House, a conductor orchestrates his own disappearance in order to take up a new vocation as a truck driver. ``A Case of Stolen Eggs'' finds Mr. Pin at the Field Museum, hot on the trail of a fossil thief. Short chapters, plentiful illustrations, and a terse style make the book a promising candidate for transitional readers. The solutions to the stories, however, are somewhat far-fetched, always hinging on a chocolate clue and relying too much on coincidence. Also, the book suffers from a case of style over substance; the mysteries do not live up to the writing, which deftly parodies the adult detective genre: ``Chicago was a big city, with slick streets when it rained hard. There were plenty of tough cases to solve. But it would take more than a little grit to stop a detective who knew his chocolate as well as he knew his city.'' --Ruth Smith, Chicago Pub . Lib .