Good news: The long-awaited last part of a trilogy, The Last Lion: Winston Spencer Churchill: Defender of the Realm, 1940-1965, is coming out later this year (and already available for pre-order). It was completed by Paul Reid, who took over the project after William Manchester's death. Some more info is here, by Steven Hayward at PowerLine.
One point: I don't agree with Hayward's contention that Manchester portrayed Churchill as a Victorian throwback, this being the key to his greatness. Very early on, in the first volume, Manchester quoted Clement Attlee's description of Churchill as like a "layer cake" with nineteenth and twentieth century slices but also earlier ones "and another, curious, layer which may possibly have been the twenty-first."