About the Related Books Widget
This widget appears only when viewing blog posts. If one of the tags of the blog post matches a tag of one of the books, that book will be displayed in the widget.
If multiple books match tags with the blog post, one will be chosen by random.
If no books match tags with the blog post, the widget will not be displayed at all.
Although not a widget, this extension also allows you to add a Related Books section to book pages. See Showing Related Books on Book Page.