Is it OK to post something like "I recently published a math book, now I am running a promotion campaign, you can dowload the book for free. I will appreciate any comments on the book, etc., etc." as a question on the MathUnderflow?
This question would not only be off-topic, it would arguably qualify as spam. Please do not post this type of content.
Of course, if in the process of writing a book an honest mathematical question arises, feel free to ask it. There is also nothing wrong with mentioning your book-project as context in this case.
Yet, the motivation for posting a question should be an actual mathematical questions not the desire to promote your work.
The same goes for answers. It is alright to mentions ones own work in an answer, as long as it is actually pertinent information and this arises organically.