I’m assuming it means mentioned as a post. I know I’ve personally seen Cal Newport’s, Deep Work mentioned in the comments at least a dozen times.
The most mentioned book all year was really only mentioned 7 times?
According to HN search, SICP was mentioned >5000 times over the past year: https://hn.algolia.com/?query=sicp&sort=byPopularity&prefix=...
So I'm guessing the number for 2017 was similar.
Without a breakdown of the method, this is just an affiliate link dump...