All the advice above is very good. Here are a few random additional thoughts.
About beta readers: these are people whom you ask to read your book and give you their responses to it. The name comes, as far as I know, from beta versions of software - ready for testing, but not for sale. In the case of written work, they’re often friends and family, but they can be people you know less well. They need to be (a) reliable, in that if they say they’ll read your book, they will (b) able to deliver criticism without offending © as representative of your intended readership as possible. However, in your case, on reconsidering, I think looking for and assigning such beta readers may be slightly premature, a task for further down the process.
About structure: I have a lot of sympathy. Structure in fiction may not be straightforward, but if all else fails, it has a basis: time. Chronology and cause and effect have a logic which you can tinker with, but which it is virtually impossible to ignore. Non-fiction structure (I think) is harder. Logic often suggests a structure, but that may not be the most interesting thread for the reader to follow. So the writer of non-fiction is confronted with all sorts of questions about structure that the writer of fiction has to worry less about.
For example, do you start with history? Or a case-study? Or the story of your own involvement in the field? Or a summary of what you intend to say? Or some of these, or all? Questions like these can cause the writer of non-fiction to end up confused and even bored (disastrously) by their own work, and to lack motivation to continue (I’ve observed).
Further down the process, it may be worth your while looking at some of Malcolm Gladwell’s books to see how he handles structure issues. Or Think Better, a kind of self-help book written by the Scrivener user Tim Hurson.
But first, your lack of urgency: my advice is to just get all the information down as quickly as you can in whatever order it occurs to you at the moment to lay it out. Just do it! Difficult, I know, but don’t worry about structure - yet. No piece of long-form writing is perfect first time: perfection is the enemy of finishing. You can always re-arrange the fragments later (thanks to Scrivener). Who knows, your instinctive structure may be the best one.
That would be much better than hiring someone to hold a metaphorical cosh over you; then you won’t enjoy what you’re doing and your writing will show it. And anyway if you do end up hiring any kind of outsider to advise you, the first thing they’ll want to see is what you’ve got to say.
One of my favourite writing quotes is from the screenwriter Robert Towne, who wrote among other famous movies Chinatown. He was nagged by a young would-be to tell him the secret of successful screenwriting. Towne gave him a small piece of paper on which were written two words --“Just finish”.