Lacuna Books allows you to invite reviewers to read published versions of your book and post comments. You can adjust your book's visibility and review option to control the level of exposure. For example, during the early stage, you can set your book's visibility to Private, set review option to Invite only, and invite just a handful of people. As it progresses, invite more people, or change review option to Open to public so that everyone can review your book (with admin moderation of course).
You can also invite co-authors to collaborate on your book. They have access to Book Editor to update text; however, only admin user can update general information, invite more collaborators, and publish the book.