The Muppets rank right up there with the 70s’ greatest gifts to humanity, but their legacy is in serious danger. Ongoing rights problems have for years prevented a full DVD release of “The Muppet Show,” and the intermittent good feelings generated by recent Muppets films only temporarily stave off one’s mistrust of Disney ownership. In the meantime, though, we have those first three seasons to savor—and, now, this exhaustive but well-paced biography, which marshals the Gonzo-like disorderliness of a life’s facts into a readable narrative with an aplomb that even Kermit, that still point in a turning Muppet Theatre, might envy. (Ballantine Books)