I also liked the Markham a lot 😀 (I ended up writing a crossover fic featuring it for Yuletide having never previously read it, solely on the basis of the requester's prompt sounding intriguing, and Isis very kindly beta'd for me).

Richard Hillary's The Last Enemy is fascinating - the memoir of a Spitfire pilot, written during the war. I was actually reading it for details about Archibald McIndoe's work (pioneering plastic surgeon) as I have been fiddling with a relevant story idea for ages, so can't remember specific details about flying, but I would recommend it and it may be on project Gutenberg.