Even mathematical books like algorithms are outdated today, for example look at the algorithms of sorting. Also methodologies and other managers literature also got changes.
So as for me another features for your recommendation system can be:
year of the book, number of recommendation on “goodreads” and rating, number of likes on the post, karma of the author of the post. Plus add weights to this data and manual evaluation of results.
Also you can add agree/disagree button to the site.