If you know that the html is actually a xhtml (an html which conforms to the xml standarts [more or less]) you can parse is simply with tools dedicated to xml (which are generally simpler than those for html).
var hrefLink = XElement.Parse("<th><a href=\"Boot_53.html\">135 Boot</a></th>")
.Descendants("a")
.Select(x => x.Attribute("href").Value)
.FirstOrDefault();