"Spitfire: Portrait of a Legend" by Leo McKinstry.
I was bought this book for Christmas and I thoroughly enjoyed. It has a very interesting account of the rivalry between Dowding and Park on the one hand and Leigh-Mallory and Bader on the other. That rivalry led to the Battle of Britain being much more of a close-run thing than it actually needed to be.
This book is a very easy read, but has been researched very well indeed. The only problems that I can see with it is the dreadful cover. The synopsis sounds like it was written by a teenage fanboy. There must be literally thousands of dramatic photographs of Spitfires that could have been used for the cover. Instead they have used a very badly Photoshopped photograph of a badly-built 1/48 Hasegawa Mk IX.
Ignore the cover. Read the book!