There's a good book about KLM's involvement in these flights, called 'Sluipvluchten naar Lissabon' by Ad van Ommen (1985), but unfortunately it is only available in Dutch and my copy is hiding in a box in the basement somewhere. I'm sure other books have covered this topic as well though, but I cannot think of a specific title right now. If I stumble across my copy sometime in the next couple of weeks, I will look up some details if you want.