It wasn’t just the detective sitting in his office at the corner of Berkeley and Boylston, munching on a doughnut and “drinking coffee with a little milk and sugar’’ that got to Joan Parker. Sure, she loved the first paragraph of the new manuscript. It was vintage Robert B. Parker, describing his alter ego, the iconic detective Spenser, and Joan had always been her husband’s first reader and biggest fan. His longtime publisher, Putnam, didn’t want to see the best-selling Spenser series die with him, nor did his family. Parker, a prolific writer, had left three books in the pipeline. But after that, who would, who could, pick up the mantle?