I don't think you'll find a LARGE youtuber on here but I could be wrong, but just in case I thought I'd put myself forward. I have a youtube audience with 10,500+ views. I have 109 followers on instagram and 55 followers on my blog. I talk about mental health and lifestyle a lot, so a self help book would fit in perfectly with my audience. If you don't find what you are looking for, I would be happy to help you out.