Non-Fiction:
High Adventure by James Norman Hall - an account by a WW1 pilot.
Falsehood in War-time by Arthur Ponsonby debunks some of the propaganda myths perpetuated during WW1
Spike Milligan's war memoirs (starting with
Adolf Hitler: My Part in his Downfall) are very good
The One That Got Away tells the story of Franz von Werra, the only German prisoner of war to escape and return to Germany after being captured by the Allies
Inside the Third Reich is a memoir by Albert Speer, Nazi Minister of Armaments from 1942 to 1945. Obviously biased, but an interesting read.
Fiction:
I highly recommend
A Sailor of Austria by John Biggins. It's about a WW1 Austro-Hungarian submarine captain. There are several books featuring Otto Prohaska, I'm currently reading the second (which is a prequel to Sailor of Austria), and which is also very good.
Das Boot (The Boat) is a very good novel about a WW2 German u-boat crew.
The Losing Role by Steve Anderson is an excellent novel about a German involved in Operation Grief, when Germans dressed in US uniforms worked behind US lines during the Battle of the Bulge.
Churchill's Queen by AJ Davidson. The blurb says "Fans of The Eagle has Landed and Foyle's War will enjoy this book". I haven't read Foyle's War but I would recommend it to anyone who enjoyed The Eagle has Landed.