At the end of the Feb. 16 broadcast of “SNL50,” a tribute to the half-century reign of “Saturday Night Live” over American culture, Martin Short thanked executive producer and series creator Lorne Michaels,

Throw in the sheer tonnage of think pieces and appreciations and other navel-gazing and you’d be forgiven for asking: Do we also need a book about Lorne Michaels? Somehow, despite all of the above, Susan Morrison’s “Lorne: The Man Who Invented Saturday Night Live” — out Tuesday — emerges as indispensable,