A new collection reveals Prince’s artistic scope at the height of his powers

It’s hard to imagine now, but when Prince’s Sign O’ The Times hit record stores in the spring of 1987, it was widely viewed as a comeback record. The 1982 double album 1999 had converted the critics’ darling into a certified hit-maker; and 1984’s Purple Rain LP and film turned Prince into a superstar. But 1985’s …