in Writing
What a subtitle does for non-fiction
A non-fiction title can afford to be intriguing because the subtitle does the explaining. How to write the line under the title.
Novels rarely have subtitles. Non-fiction books almost always do. The difference is not a matter of fashion. It reflects a real division of labour.
A non-fiction title often has two jobs that pull in opposite directions. It needs to be memorable and appealing, which favours short, suggestive phrases. It also needs to tell a browser what the book is about, which favours plain description. A subtitle lets the book do both.
The division of labour
Consider a hypothetical history of salt. A title such as White Gold is intriguing but ambiguous. It could be about porcelain, cotton or a football club. A subtitle such as How Salt Shaped the Ancient Mediterranean removes the ambiguity at once and tells the reader the scope and the angle.
This pairing, an evocative title with a descriptive subtitle, is the most common pattern in general non-fiction. It lets the title be short and characterful while the subtitle makes the promise.
What a good subtitle says
A strong subtitle usually answers some of these questions. What is the subject? What is the scope, in time, place or range? What is the argument or angle? Who is it for?
It does not need to answer all of them. A biography might simply give the subject's name and dates, or a phrase that captures their significance. A book of ideas might state its argument outright. A practical book might say what the reader will be able to do.
The key is that the subtitle should make a claim a reader can evaluate. Vague subtitles, the kind that could sit under almost any book on the subject, waste the space. A specific one tells the right reader that this is the book for them and tells the wrong reader, fairly, to keep looking.
Common problems
The most common problem is length. A subtitle that tries to list every theme of the book becomes a paragraph and stops working as a line. A reader should be able to take it in at a glance on a small thumbnail as easily as on a shop table.
The second is overpromising. A subtitle that claims the book will explain everything about its subject, or change the reader's life, invites disappointment and scepticism from reviewers. Accuracy tends to sell better over time than inflation.
The third is repetition. If the title already says the book is about salt, the subtitle need not repeat the word unless it adds something. Use the space for what the title leaves out.
Search and discovery
Subtitles also help readers find books. People search online for subjects rather than titles when they do not yet know which book they want. A subtitle that contains the words a reader would search for makes the book easier to find. This is a reason to prefer plain language in the subtitle even when the title is playful.
That said, a subtitle written only for search engines reads badly. The words should be the natural way to describe the book, which is usually also the way people search for it.
As with titles, try subtitles on people who have not read the book. Ask them what they expect it to contain, and compare their answer with what the book actually does. Where the answers diverge, the subtitle is promising the wrong thing.
A good subtitle is rarely quoted or admired. It is simply the line that tells the right reader to pick the book up.