Complex sentence: The Great American Songbook is a canon of the most important and most influential American popular songs of the 20th century principally from Broadway theatre, musical theatre, and Hollywood musical film, from the 1920s through the 1950s, including dozens of songs of enduring popularity.
Split into simpler sentences: The Great American Songbook is a term used to denote the canon of the most important and most influential American popular songs of the 20th century--principally from Broadway theatre, musical theatre, and Hollywood musical film. These enduring songs are from the 1920s through the 1950s, and include dozens of songs of enduring popularity.