"progressive spiritual orientation" ... Both adjectives are loaded with unsupported assumptions about good/bad, true/false, and have a connotation of activism/dogmatism. You may have a great book there, but I wouldn't give it attention just because of that description. On the other hand, I believe there might be others who would be interested in it just because of those adjectives, as they consider themselves progressive and spiritual, by their own definitions.
It is also surprising that you firstly wrote your book ("I've written a self-help book ..."), and now you're looking to back/pad it with celebrities.
Are you a published author? If so, what have you already published?
By the way, many A-listers swear by TM (transcendental meditation), e.g. Jerry Seinfeld attribute his successful career to that. But, that might not be progressive spiritual ... (?)
Everything expressed here is what I believe. Keep that in mind when you read my post, as I kept it in mind when I wrote it. I don't parrot others. Most of my spiritual beliefs come from direct channeling guidance. I have no interest in arguing whose belief is right, and whose is wrong. I'm here just to express my opinions, and read about others'.