Jonathan Karp is “a baby-faced 40-year-old” who became editor-in-chief at Random House in May. He has a glittery list of authors, and Variety has also called him “one of the more visible book publishers in town.” “Still. For years and years and years, like scads of lesser-known schleps who wait tables or answer telephones, Mr. Karp dreamed of writing a musical comedy, a dream he has nourished through playwriting workshops, amateur readings and weekends and vacations devoted to lyrics and plot lines.” So he did…