Lots of books but most will be out of print now.
Britain's Imperial Air Routes 1918-1939 by Robin Higham.
Merchant Airmen published by HMSO covers the war years.
From Flying Boats to Flying Jets by Eric Woods(a BOAC navigator 1946-72)
Both Feet in the Air by Archie Jackson(a pilot with BOAC/BSAA/BOAC again)