Baraka_Guru

It's a good idea to get interviewees to sign a release form.

Here's one example:
Sample Information Release Form

You could customize this to the details of your project. Basically, anyone who signs it agrees to be interviewed and that the information obtained from the interview can be used by you in the publication of the book.

So long as you have concrete details: book (working) title, author's name, etc., then the forms can be presented to the publisher, or you can keep them on-hand if you're self-publishing.

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Knowing that death is certain and that the time of death is uncertain, what's the most important thing?
—Bhikkhuni Pema Chödrön

Humankind cannot bear very much reality.
—From "Burnt Norton," Four Quartets (1936), T. S. Eliot