I am sure that writing the book will not be an easy task, if done properly. I am not an historian so cannot help with the recommendation of any books of an historical nature.
However, here are some of my initial thoughts:
1. Define the purpose of writing the book. Is it purely for history? Is it for profit? Is it both?
2. Define the audience that you are writing the book for. Is it only for the families? Is it for scuba divers? Defining the audience will determine how the book is written. If it is only for history, then the book will be filled with facts and pictures. If it's for divers and the families, then a story, based on the facts, can be woven in.
3. You should have no problems finding research info. If the sub was sunk by the Italians, then they most likely have some historical information somewhere that will help develop the story. The research, in this case, is not about submarines in particular but about the naval war.
4. If you haven't already read "Shadow Divers", which is an account of the discovery of U-869 off the coast of New Jersey here in the U.S., you should. The book is very interesting and may give you some ideas.
5. Give thought to having a professional writer actually write the book.
I wish I could be of more help immediately. I'm sure if you ask here on Scubaboard for advice, you will find someone who can help even more.