By a weird coincidence, I just bought Lerche's book last week at a used book store, having never heard of it before. It's a fantastic read.The book that Peter mentioned is -
"Luftwaffe Test Pilot" by Hans-Werner Lerche.
Two others that may also be of interest are -
"In Enemy Hands" by Bryan Phillpott
"Foreign Planes in the Service of the Luftwaffe " by Jean-Louis Roba.