The book is referring to an external site http://www.sql-join.com stating “If you need a primer on joins (or even a refresher), the SQL-Join website provides some detailed examples.”
However, this website seems to no longer exist and gets redirected to a company website https://www.talend.com that does not explain anything about SQL or Joins.
I would suggest linking to the Wikipedia page https://en.wikipedia.org/wiki/Join_(SQL) or other stable resources instead.