With regard to the back link I would suggest that the text in the book itself be used as a back link. For example if the index entry was or Earasaid then the entry in the book itself would be <a id="Index001" href="../Text/Index.html#earasaid">Earasaid</a>
The index entry would refer to the id Index001 while the back link would be visible via the highlighted word Earasaid in the text. This does two things. The text being highlighted would indicate an Index entry which would make it easier to find if you click on the entry in the index and when you see it in the text you would know it was indexed and could then follow the link to see if there are more references in the text for that word. (note the double use of the <a to provide both the id for the index link and the back link to the index.)
Dale