Do a survey of present day Cirque du Soleil shows all over the world so you can establish a starting point to work backwards from. Visit the Cirque du Soleil website, watch DVDs of the various shows and read newspaper reviews about openings of the famous circus.
Learn about the two founders of Cirque du Soleil, Guy Lalibert and Daniel Gauthier, to get a personal history of the circus. The two partners started Cirque du Soleil when they were just in their twenties and facing failure and rejection by audiences and the bank. Read about the rise of the two entertainer-entrepreneurs who went on to become two of the most successful businessmen in the world.
Reference one of the many "biographies" of the circus such as the authorized, "Cirque du Soleil: 20 Years Under the Sun," "Wings: Backstage With Cirque du Soleil" or "Cirque du Soleil: The Spark" for an excellent cumulative history of the fantasy circus.
Watch Cirque du Soleil documentaries like "Cirque du Soleil, the Fire Within," which follows a number of Cirque du Soleil performers through their daily routine. Also take a look at "Cirque du Soleil, A Baroque Odyssey," which takes a broader look at the circus and places it in a historical and aesthetic context.