Book excerpt: On January 12, 1926, radio audiences heard the first exchanges of wit and wisdom between "Sam 'n' Henry"--the verbal jousters who would evolve into Amos 'n' Andy and whose broadcasts launched the radio sitcom. Here is a detailed look at 20 of the most popular such sitcoms that aired between the mid-1920s and early 1950s, the three-decade heyday of radio. Each series is discussed from an artistic standpoint, with attention to the program's character development and style of comedy as well as its influence on other shows. The book provides complete biographical profiles of each sitcom's stars as well as several actors whose careers consisted primarily of supporting roles. Appendices include an abbreviated summary of 13 sitcoms beyond those discussed in the main body of the book, and a comprehensive list of 170 radio sitcoms. Notes, bibliography, index.