Wow 400 pages, I hope it's all your own work/words. You don't want any accusations of plagiarism following your book around.
But, as Samer mentions. I've been quite surprised how supportive Amazon appear to be for printed versions of previously unknown writers. I've even purchased a few books on specialist subjects from basic atom and electrical theory to the nature of quantum theory and applications.
These books tend to be short, around a 150 pages or less and many are local publications Doctorate thesis etc translated in English. But it shows you can find an outlet.
I don't think it's a good idea to be 'Promoting' a book before you have at least a hard copy of the finished text in hand, preferably in its bound and saleable format. It going to read too much as a speculative venture and book buyers are impulsive buyers (even for future publications) For a factual book your going to be Quoting contents and examples for it to be taken seriously. Also picture/s of the finished bound work goes a long way.
Expect your background and expertise to be truly scrutinised and commented on by fellow domainers