The summary property is intended to be used for fallback for when the server doesn't recognize the other properties of the request. Most Micropub servers understand content and photo, but this is a great example of an interesting content type that is likely not supported by most servers yet. In this case, summary is the correct place for this fallback content. The content property would be misleading since normally the content property contains user-entered text. For example, if the user enters a note about why they wanted to read this, that could be included in the content property.
I don't think this really solves the problem, though. If a server implementation prefers content over summary (which it should), with this example the context of the book title will be lost unless the server is updated to understand read-of.
