So much weight lies on the few words that must:
1.. Sum up what the book is about
2. Be a hook for the right audience you are targeting
So what is the right approach to chiseling out a title that does both? Coming from SEO background, I do tend to depend on search data to be an important part of the book title engineering. So, I would advise use that information as the starting point. Once you have narrowed down and shortlisted 2-3 phrases, choose two to make one the main title and the other the subtitle.