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My cousin is an extraordinary little girl from an extraordinary family. She was featured last Christmas on NBC Nightly News.
I have discussed with her about writing a book about their story because it needs to be told and people need to read about it. I write pretty well but don't have a publishing background and they don't have anyone seeking their story rights.
I was thinking about offering to write the book on spec optioning their rights on spec for something like 6 months and then we would each receive half of the profits of the book.
Does anyone have any experience writing books in this situation? Does this seem like a fair deal on both sides?
A fair deal on both sides really depends on the sides. To me it seems fair. But to them it might not. So the best thing to do would be to discuss it with them. A person like your cousin or her parents (just an example here; nothing personal) might say that since this is the child's story, they might deserve more than half the profits. Or they might say that you should agree on a price beforehand and stick to that. There are many different options here and I doubt anyone can actually tell you what you should do, besides discussing it with your cousin (and her parents if she is underage).