Find a professional editor to read your work and to offers suggestions. It's their job and they will offer objective criticism and it will serve you well.

Never have a friend read your work. That is, not if you want to continue your friendship. There are a number of reasons for this, not the least of which is the sort of honest interchange that must take place for such an endeavor to be productive.

You are in the right place with this site and any other site that caters to or supports folks who aren't god-ridden or religion bound. That is but the first step. I suggest that you read this article by Jane Friedman before you do anything further. Chew on what it is she has to say first. I don't say her word is the definitive one or that it is the only one, but it is a good one. After reading this article, you might have a different outlook on this issue of 'test audience.'

[janefriedman.com]

It can be helpful to have it looked at by friends, or preferably, more than one friends, but it needs to be a suitable friend. I got friends to read and criticise my book about assisted dying, but I made sure they were not upset by the idea first. I didn't ask my closest friend who has fairly recently come close to death! Don't have it professionally edited unless you want it to be written in someone else's style!

Absolutely get someone -- or more than one person -- to read your manuscript. I've read and reread the work of two writers before publication. I can guarantee a second or third pair of eyes will improve the product. give me a telephone to chat, I can tell you horror stories. I was a newspaper reporter and copy editor for decades

It's always better to have feedback on writing before sending it off, especially if it's a self-publisher which tend not to have large editing staffs. Also, you already know it's a challenging read. Feedback may include suggestions for lightening it. Find out if there is a writing community near you and see if you can join a critique circle that accepts nonfiction. Failing that, see if you can find an online group.

It will probably take some time to get the whole thing workshopped. In the meantime, find out if the publisher you are looking at will evaluate book proposals in addition to over-the-transom manuscripts. That might be a way of learning whether your manuscript is a good fit with that publisher before you go through a lot of workshopping and revising. Good luck!