You should not use heading tags for things that are not headings.
I use something like:
<p class="title">Book Title</p>
<p class="subtitle">Book Subtitle</p>
and style p.title, p.subtitle, p.author, and p.date the way I like them. Also, I usually have a specific CSS file for the title page, so it doesn't interfere with the main book styles, and I can share it easily with other books.