The book’s title is implicitly at the <h1> level, even if <h1> is not present in the ebook. An <h1> element is only present if the ebook contains a half title page. Because of the implicit <h1>, all other sections begin at <h2>.
But 6.3.3 says that the title page uses <h1> and 6.5.4 says that the half title page uses <h2>.
Section 7.2.3 says:
But 6.3.3 says that the title page uses
<h1>
and 6.5.4 says that the half title page uses<h2>
.