I have look around on the internet and really have not found much. I would begin by discussing it with the author. I think if he is well known, he will be more than willing to work with you.
After all he is the expert and I don't think he will try to take advantage of the situation. I also think it is important to take into consideration,what will you and your family benefit from such a unique opportunity? Imagine your daughter will be on this cover for years to come, not everyone has that opportunity.It will be something that she can keep, cherish and share with her family when she is an adult.
Good Luck!
Talk with your friend and ask him/her what usually pepole charge. I mean he/she is your friend. Just ask!
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.