Inclusion of a subtitle in your book's data is a requirement of our North America distributor, NBN. This does not have to be included on your book's cover. If you would not like your book cover to have a subtitle, you can note this in your proposal. However, if you're offered a contract and you accept it, the decision ultimately lies with the publisher.
If a subtitle isn't included in the cover design one will be added by the sales co-ordinator at the point of distributing the title metadata to the trade, and it will appear on sites selling your book online. It will also be included with the rest of your book's data in catalogues used by the Sales Teams. We may also add or edit the subtitle on Amazon at a later date to reflect key marketing information such as a prize-win.
A subtitle is particularly helpful in non-fiction, as it can help trade buyers and readers to instantly identify the book's theme and the market. When a book appears on Amazon in sections such as 'Customers also bought' and 'Related products', having a title followed by a subtitle provides clarity about the book's subject matter and encourages click-through to your book. Visibility is key.
On the cover, subtitles are usually in lower case, except for the first letter of the first word, even if the main title is all in upper case. On the title page, the subtitle is lower case.