Hi again OP
Some suggestions for your dad:
That Summer by Andrew Greig. The story of a young pilot and a female radar operator during the Battle of Britain. (I think your mum might also like this).
All the Light We Cannot See by Anthony Doerr. A blind girl and a German boy trying to navigate their way through the chaos of WWII in Europe.
Everyone Brave is Forgiven by Chris Cleave. London during the Blitz and the story of a young woman charged with teaching hildren who weren't evacuated while her boyfriend is abroad fighting ( one or two distressing scenes.)
Charlotte Gray by Sebastian Faulks. A young woman working in mainland Europe as an SOE while also searching for her missing fiancé.
Enigma by Robert Harris. The development at Bletchley of the amazing machine which contributed so much to the defeat of Nazi Germany. Some technical/maths stuff which I had to skim a bit but still extremely readable.
Fatherland also by Robert Harris. Imagining how the world would have been had Nazi Germany won the war.
And some further ideas for your mum:
The Shardlake series by CJ Sansom. Tudor mysteries.
The Inspector Gamache series by Louise Penny. Gentle murder mysteries set in a cosy village in French Canada.
Alexander McCall Smith's The No 1 Ladies' Detective Agency set in Botswana. Again, nothing grisly or disturbing.
Have a look OP and see what you think. And if you need more ideas, you could ask MNHQ to move this thread to What We're Reading as you'll get some good suggestions over there.