A subtitle is helpful in non-fiction.
Particularly if you’re a first-time author and readers don’t know what to expect from the book, it can help to clarify the theme and the market. The average buyer in a chain bookstore looks at a book for three seconds before moving on. If someone is searching Amazon for a subject, the right subtitle helps them find it. Look for broad suggestions for keywords at www.wordtracker.com. Fine-tune them at inventory.overture.com. Or check out Roget’s Thesaurus.
On the cover, subtitles are usually in lower case (except for the first letter of the word) rather than all upper case, even if the main title is all in upper case. But it can vary; it is left to the designer. On the title page, the subtitle is almost always lower case rather than upper case.
Please note: we cannot change the subtitle once the cover is finalized and information has gone out to the trade.