A must-read is
Aircraft Down! Evading Capture in WWII Europe, by Philip D. Caine. The main focus isn't the actual flying, but what happened AFTER pilots were shot down. Each chapter chronicles the evasion experience of a seperate pilot, and it is truly amazing to read of the many ordeals they went through to reach England again; none of them could have done so without the help of the French, Belgian, Greek, and Italian underground systems.
A book that I haven't read, but was suggested by a Pole in the Subsim Polish Language Forum is called
A Question of Honor The Kosciuszko Squadron: Forgotten Heroes of WWII, by Lynne Olson and Stanley W. Cloud. It is about 5 pilots of the Polish 303 Squadron during the Battle of Britain -- many people have said that without the 303, the BoB could not have been won. It looks like a great read, here's the website:
http://www.questionofhonor.com/index1.htm