What do you call the second part of a Book Title?

There isn't a specific term for the second part of a book title. However, here are some ways to think about it:

* Subtitle: This is the most common term for a second part of a title, especially if it provides additional information about the book's content or theme.

* Sub-title: This is an alternative spelling of subtitle.

* Secondary Title: This term emphasizes the second part as a distinct element of the title.

* Colon: The presence of a colon often indicates a subtitle, though it's not a strict rule.

Examples:

* The Lord of the Rings: The Fellowship of the Ring (subtitle)

* Sapiens: A Brief History of Humankind (subtitle)

* The Hitchhiker's Guide to the Galaxy: The Restaurant at the End of the Universe (secondary title)

Ultimately, the specific term used depends on the context and the book itself.