There is a version number at the top: "v8.8.0.8". I believe they keep making updates to the book probably to fix typos/errata. The original link I had bookmarked https://htdp.org/2018-01-06/Book/index.html and this one look mostly the same - content wise.

But it would be nice if there was a CHANGES file or description about what does change between these versions.