The most commercially successful pop group of the 1970s, the origins of the Swedish superstars ABBA dated back to 1966, when keyboardist and vocalist Benny Andersson, a onetime member of the popular beat outfit the Hep Stars, first teamed with guitarist and vocalist Bjorn Ulvaeus, the leader of the folk-rock unit the Hootenanny Singers. The two performers began composing songs together and handling session and production work for Polar Music/Union Songs, a publishing company owned by Stig Anderson, himself a prolific songwriter throughout the 1950s and 1960s. At the same time,...