The href="#book_title" works if the target is in the same file inside the .epub. Otherwise, you need to give the path to the other file, like this:
In toc.xhtml (for instance):
Code:
<a href="book1.xhtml#book_title">Go to Book Title</a>
(this says: go to file book1.xhtml and there find the element with id="book_title")
and in book1.xhtml:
Code:
<h1 id="book_title">Book Title</h1>
It should not matter (for the link to work) whether the element is an <a> or an <h1>, unless something is really buggy in between.
Validating with FlightCrew (in Sigil) should tell you if there's anything wrong.