How to Write A Subtitle

It's hard to get the whole idea of a book into the few words of a book title--which is why someone invented subtitles.

Subtitles are often used to clarify or expand on the title of a non-fiction book (novels generally do not have subtitles). Titles that involve unfamiliar terms, or that obliquely reference book contents with jargon or a lyrical passage generally benefit from a strong, clear subtitle. Some examples are:

• SEAL Team Six: Memoirs of an Elite Navy SEAL Sniper - The subtitle elaborates on the somewhat unfamiliar terminology in the title
  

• A Clearing in the Distance: The Biography of Frederick Law Olmstead - The book title is a lovely but not-very-explanatory line taken from the book; the subtitle explains what the book is.
  

• The Millionaire Next Door: The Surprising Secrets of America's Wealthy - The title is intriguing, but it could be a biography or a memoir or a book about real estate. The subtitle gives the book mass appeal: who wouldn't want to know those secrets?

• Eat, Pray, Love: One Woman's Search for Everything Across Italy, India and Indonesia - Before it was a huge bestseller, Elizabeth Gilbert's book benefited from a strong subtitle.
  

While even a very book good title can benefit from the further explanation a subtitle provides, some need no additional explanation. For example, the book John Adams was written by an extremely well known and identifiable biographer, David McCullough. There's no subtitle on the jacket; the book needs no additional information.

For more about writing good book titles, check out:

What makes a good book title?
Good Book Titles: Step-by-Step
How to make a book title better: case study for a selling book title