The Splendid And The Vile, Eric Larson

 Here is NPRs review of the book. 

If you haven't read lot of Churchill books, this is a good one to cover especially the early war years. Larson is very readable, and tends to approach things from somewhat of a "woman's perspective".  Romances, affairs and unrequited loves are documented -- not the typical fare of such books. He also tends to go more into the relationships between the players -- in some ways it reads more like a novel than a historical book. 

As in any decently written Churchill book, it makes it clear that Winston was the indispensible, very unique, and somewhat eccentric man.