I've written an online book. When I search for the book on Google, it is labeled as <no_title> rather than the title in the HTML's
<title> tag. Any idea how I can update the title in the Google search? Thanks!
Check Google's cached version - you can see that "<no title>" was actually the title of your page at the point it was crawled in June:
<title><no title> — Machine Learning from Scratch</title>
Use Google Search Console to get Google to re-crawl your page, and it will see and index the new title.