The Gathering Storm is the first book in Churchill's six volume history of WW2.
For a while this was the definitive history of WW2: no longer. Churchill had to keep some things secret (such as Ultra and Bletchley Park) when he wrote this, so there are gaps. But it is the only personal account by any of the 'Big Five' leaders in that war and as such it's still a very important source for anyone reading up on this area of history.
In this first volume, Churchill covers the Interwar years from his own viewpoint. He became convinced very early on that Germany was set on a path that could end in another World War. While it's common to portray him as 'a voice crying in the wilderness', his access to high level government information (shown in this volume) makes it obvious that many of the political leaders and civil servants had a much higher regard for him and his views than they could afford to publicly express.
The second part covers the early war months, where Churchill was in charge of the Admiralty and the sea war - the most active theatre of operations in the 'Phoney War' period. The volume ends when Chamberlain's government falls and Churchill becomes Prime Minister.
As well as being an M.P., Churchill was in the 1920's and 1930's a well-regarded writer and popular historian. While his style is of his time, the book is both logically presented and very well written. It also contains a large amount of Churchill's own letters and memoranda, so it's full of primary source material.
I do have one minor niggle with the Kindle edition: the proof reading wasn't very good. There are more than a few typographic errors - which at times became a bit irritating.