Most Dangerous Enemy - Essential reading. Best BoB book ever in my view, covers every aspect from leadership to technology to manufacturing output to aircraft to politics to the men themselves, in a very objective but easily readable way.
Fighter boys - also very good book focussing mainly on the men themselves, from Albert Ball, Mannock, McCudden to Malan, Stanford Tuck and many lesser known pilots, many of whom were killed, but left diaries.
First Light - Nothing need be said here I think.
Johnny Kent - 'One of the Few' great stories of building his own aircraft as a youth in Canada to death defying Test Piloting at Farnborough to leading 303 Polish Squadron, 92 Squadron and a lot of interesting post-war stuff.
Spitfire Story - Alfred Price, if you want the story of the aircraft itself.
Not read Bob Doe's book or Brian Kingcombe's and many others but will get there eventually!
Derek Robinson touched on his views on the invasion in his novel 'Piece of Cake' but I've not read Invasion 1940.