In Monkey Trouble, book 127 in the Boxcar Children Mystery series, the  Alden children, Henry, 14, Jesse, 12, Violet, 10, and Benny, 6, spend two days touring a zoo to see the new baby animals.  They buy a camera to share and enter a contest for the best photo of the animals.  During the tour, a camera goes missing, a Capuchin monkey escapes from its cage, and suspicions and mysteries abound.  By recalling memories and making astute observations, the Aldens find answers to each problem encountered.  Monkey Trouble is typical of the many stories written and inspired by Gertrude Chandler Warner.  These chapter books are written at a third or fourth grade reading level and relate mystery-solving adventures of the self-sufficient Alden children.  Most chapters contain a black and white pencil illustration by Robert Papp.