The Phantom Airman by Allan Frewin Jones

I read about The Phantom Airman by Allan Frewin Jones. It's central characters are Regan and Frankie and Jack and Tom. They are same central characters of Unquiet Graves which I read last week.

After Reading: Terry Bowles read his grandfather's diary and he discovered something - his grandfather was a traitor. He was a spy for the Germans and they gave him diamonds to spy other code which prove that Squadron Leader Leighbridge-Smith was a traitor.

Code : K co urv hqt vjg Igtocpu. K dtqmg Ingp'u rncpg uq jg fkg. K mknngg Ingp. K iqv ocpb fkcoqpfu hqt kv'u rcaogpv. K co iqkpi vq ngcxg hqt oa fguegpfcpv.

Answer : I am a spy for the Germans. I broke Glen's plane so he die. I killed Glen. I got many diamonds for it's payment. I am going to leave for my descendant.