One of the first books on this theme was David Masters' 'German Jet Genesis', published by Jane's in 1982 (ISBN 0 7106 0186 7). Since then there have been many What If? titles about German experimental aircraft. One I can recall from recent years was 'Luftwaffe Secret Projects: Fighters 1939-1945' by Walter Schick. I think Midland Counties Publications put out an English language edition about five years ago. Try
www.midlandcountiessuperstore.com.