If there's a really significant update I actually will email everyone who owns the book about it (I did that once when I added some extra bonus content).

Generally though like for typo fixes and stuff I just updated the master file, and if you redownload it you get the new one.

One of the (many) improvements to the site I have on my to-do list is for your my library to show like a "last updated" date or something, or maybe have a better curated changelog.