When playwright Terrence McNally was asked nearly two decades ago to write the book for a musical version of “Ragtime,’’ he immediately said yes.

That night, McNally began rereading E.L. Doctorow’s 1975 novel of early-20th-century America, just to be sure. “The book is so musical,’’ he said recently in an interview with the Globe. “It just sings, every page.’’