If you intend your blog to be connected with your author/book website, I think the subject matter should at least loosely connect with your writing. For example, if you write classic fantasy, a blog post about real-world Medieval weapons would not be amiss.
I have a blog connected with my series website: https://shardsofsevia.wordpress.com. In addition to bits about my books and characters, I post links to articles about words or language that catch my eye, writing tips (that I created myself) and the occasional book review of other Christian authors I’d like to support. Right now I’m working on creating a guest-post series about writing inspiration.
I’ve seen other bloggers do author interviews, book spotlights or previews, and inviting other bloggers to share guest posts about all kinds of (related) things. Hope this helps!