I've only written a book, enough to know how much work it is. Before writing it, I used to think that writing was going to be the most time-consuming task. However, it turned out that I spent more time fixing mistakes, reading again and again what I wrote and, specially, applying changes based on my dear reviewers' feedback. The first draft I wrote had nothing to do with the final version. The reviewers where friends as the book was self-published. And reviewing was hard to them. Some of them stopped following revisions as the chapters were mutating.
As a reader and also as a reviewer of other books, I like to give feedback to the author once the book is in a very advanced stage. Once the author has read his book so many times that she is tired, and she thinks it is ready to be published. At that point feedback is more precise and changes are easier to apply. Less work and better experience for everyone.
Guess what my advise is?
Do not invite your reviewers to read your book when you consider it is 50% done. Wait until you think it is pretty much done. When you consider you've got all the chapters in and you are happy with the content. At this point is when you really got 50% of the job done, but you believed you were 90% 🙂
Well, if you have been writing for many months and the book is going to be huge, then you need feedback sooner. But for the average 200-300 pages book, I would rather wait.
You can ask your reviewers about the general table of contents, or maybe about some technology your are talking about, but going through the whole book soon is not a good idea in my opinion. As the book evolve, your reviewers will be tired. And worse than that, some of them might have a wrong idea about the final version, just because they left the project too early.
Care about your reviewers' and readers' time as much as you care about your own time 🙂
And remember, never write a book for money. Write it because you feel like sharing with others. If the book is good, the reward will come, but not in the form of direct money from the sales.