We have finished our nonfiction, self-help, book draft. We require finding a mentor for improving the book's structure and for publishing and marketing. How can we find a mentor (not proofreader)? Is there this kind of service?
There are certainly people who will perform a substantive edit on a manuscript and give you an evaluation of it. They generally advertise themselves as editors or literary consultants.
However, finishing a non-fiction book before you look for a publisher is generally not the right approach. Publishers buy fiction based on the specific merits of the manuscript, so you have to finish your novel before you start to market it. But with non-fiction, publishers want to know what the market for the work is, what the competition is, why the author is qualified to write the book, and what standing they have in the community that will help sell the book. All of these are things that you can establish before you write.
If the publisher accepts your proposal, they will then become your mentor through the writing process, guiding you to create the kind of book the publisher thinks they can sell. I have written three non-fiction books and contributed to two others and in every case, the books were sold before they were written, and in every case but one the publisher provided support and guidance through the writing process.
So, you might want to take a step back and start approaching publishers with a proposal that outlines the market for the book, the competition, and your qualification to write it. (Most publishers will provide a detailed description of what they want a book proposal to cover.) You may then find you have to rewrite to suit the publisher's requirements, but at least you will have a mentor, in the form of your editor, to help you do so.
Congratulations on finishing your draft!
I think Mark has some valid points and much more experience than I have publishing non-fiction/self-help books, but this site may assist with your search https://reedsy.com/
You can also try looking for an agent on Twitter, Facebook or a writer's union/writer's guild in your country as I understand they can be helpful for navigating the industry.
Hope this helps and happy writing :)