Sending your book out for feedback or comments is a brave act and reviewers of all kinds can sometimes forget the impact their words can have. However, getting feedback can also be enormously rewarding and encouraging so it is worth the risk. The feedback you get is often only as good as the feedback your request.
The revision stage is there for you to sort out glitches, move things around, delete things and add things. If you get bogged down in this while you are writing, you will never finish. Even if you end up deleting material, it’s fine. You can use that material as the foundation for another book, for articles or even for a related product.
Books on the same topic are being written all the time. It is nuances like the design of the cover, the content, style of writing, expertise of the author, length of the book – and a whole slew of other factors that makes one book more appealing than another to a particular set of readers.