added a comment - - edited
Can I confirm what is meant by a "unique HTML title tag"? You don't really mean the <title></title> tag right? If you do, then I believe that is already present. The "<title></title>" of each book chapter page has a unique tag.
If you mean the actual content of the chapter on the page, would enclosing it in a <span> tag with a unique "title" attribute do the trick?