It would need to be added to the supports
parameter of the register_post_type
arguments in class-book-review.php
.
I’m still not entirely clear on what you’re thinking with the comments. You say “I’m trying to make your plugin like Libraries are online” — if I go to a library web site, and I pull up a book, I generally can’t add a comment on that book.
However, if I go to someplace like Amazon.com and I disagree with a particular review, I can post a comment on that review, so I can see the value, I’m just not sure how I feel about it in this context. If I understood what you were doing a bit better, I might be swayed to add support (at least as an option).