Actually, the subtitles tell you the content. The titles themselves are still more marketing labels that look good 6" wide in bold. They may relate metaphoric

Actually, the subtitles tell you the content. The titles themselves are still more marketing labels that look good 6" wide in bold. They may relate metaphorically to the content, but they would be scrapped in a heartbeat if they didn't sound sellable. In fact, given that most non-fiction books are sold to the publisher before so much as a chapter is written, it's not unreasonable to say that many such books might not even exist without an edgy and sellable title.