In April 1974, 500 million television viewers across Europe witnessed the bizarrely thrilling sight of four garishly-dressed unknowns from Sweden storm their way to victory in the Eurovision Song Contest. The song was 'Waterloo'. Abba had arrived.Over the next three decades, the band moved on through an almost unbroken succession of hit albums and singles. Abba have sold some 400 million records around the world, and their songs inspired a musical which since opening in October 2001 has been seen by more than 10 million people. In Abba: Unplugged, Karl French, a journalist and author...