Legal aspect of gathering "testimonies" for a book

I’m toying with the idea of writing a book that would be a compilation of different missionary stories– how they got to the field, made decisions, etc.

Does anyone have any insight on solidifying the legal side of this idea?

For example, having them sign a waiver that they have agreed to let me use their story, they approve the final version of their story, that I keep all royalties from the book sales, etc.

Has anyone done this? What legal aspects am I not thinking of?