For the next hour and a half, he signed 141 copies with blue Sharpie pens, fortified by a mug of coffee that a museum staff member placed in front of him. The mug, which the museum sells for $19.95, boasts in all caps: “I Finished The Power Broker.”Caro usually dislikes cracks about the book’s length. But he seemed delighted by the mug.“Did you see this?” he asked, holding up his coffee.“I’m not supposed to say this,” he said, “but I kind of like it.”