One cold December morning, Dori HadarDJ by night, criminal investigator by daywas digging through crates of records at a flea market in Washington, D.C. There he stumbled into the elaborate world of Mingering Mikea soul superstar of the 1960s and '70s who released an astonishing 50 albums and at least as many singles in just 10 years. But Hadar had never heard of him, and he realized why on closer inspection: every album in the crates was made of...