I think it would be inappropriate for a Q&A site, in the form you describe.
It basically violates every rule in the help center article on What Not to Ask. That's not to say we can't decide to break those rules, but I think we need a more compelling reason than "this is interesting and useful content" - I think it's worth breaking those rules only if "this is interesting and useful content that is helped by Q&A format."
I think this is a case where the SE format hurts, rather than helps.
- This site isn't really suited to long-form writing.
- The cornerstone of the SE model is voting. This works well, because we have a solid base of users with expertise on Academia, and we accept questions about Academia, so many of our users are qualified to vote on many of our questions and answers. On book reviews, only users who have read the book will be able to vote on the review in any meaningful way.
- Honestly, I don't see really see a difference between allowing reviews of books, vs. allowing reviews of universities, journals, conferences, etc. (Those can also be based on evidence, but we specifically disallow those here as "shopping questions.") I definitely don't want to start allowing questions asking for reviews of universities, journals, conferences, etc., for all the reasons discussed in previous meta posts on this subject.
- We don't really have a good mechanism for shutting down opinion-based answers. All we can do is shut down questions that are likely to lead to opinion-based answers. I think that book review questions are likely to lead to opinion-based answers (even if it's possible to write an evidence-based answer as well).
This kind of content is better suited for a blog format. We've already seen the negative side of users trying to use this site as a platform for their opinions and ideas. I do not want to invite more of that.
Having said that, if this community wants an Academia.SE blog for book reviews and related non-Q&A content, presumably it could happen.