There are two stories packed inside the massive third volume of “The Last Lion,’’ the breathtaking biography of Winston Churchill started by the legendary William Manchester and completed, after his death, by Paul Reid. Two stories — both dramatic, both poignant, both unforgettable.

The first is the story of the book. Manchester wrote two volumes of his Churchill triptych and had made strides toward the third when he suffered two strokes. It became evident that he could not continue a work that, more than his account of the assassination of John F. Kennedy (“Death of a President,’’ 1967), his history of modern America (“The Glory and the Dream,’’1974) or his biography of Douglas MacArthur (“American Caesar,’’ 1978), would be his highest achievement. He turned to Reid, a relatively unknown Palm Beach Post feature writer, who picked up the baton and, it must be said, finished the race with agility, grace, and skill.