I think a read post primarily indicates an action related to reading it. “I want to read this,” “I am currently reading this,” or “I finished reading this.” I think any of those could include an optional comment. The Goodreads status update UI is a good example of this: you can update your reading progress and include a comment each time. Once you’ve marked a book as finished, you have a separate UI to enter a review and a rating, independent of your previous status updates and comments.
I think the general indieweb workflow for this would be several posts:
Personally I don't think updating existing posts is a great idea in this workflow, but I'm open to ideas. These are my suggestions based on experience with Goodreads.