I want to create a free and open-source (licensed with GNU (A)GPL v3) using the plot and maybe the title of the book. I won't sell it and will have no income from it. I have some questions:

1. Should I ask the heir of the book author (he died) for permission even if I won't sell the game? If no, what law/international convention does allow me to do so?
2. If I have to ask, would it be enough to get permission via email or should I sign a contract with the right holder?
3. If I got permission for non-profit use of the book plot (or if I don't have to get that permission), can I collect non-binding donations on the game's website?

Thank you very much.