What's an introductory part of the title of an article/book called?

Solution 1:

It is my understanding that what you have bolded is called the title and what follows the colon is known as a subtitle.

See this entry from the Library of Congress: http://www.loc.gov/nls/other/annotation/subtitles.html

And see this entry from the Yale College Writing Center:

Titles and Subtitles

It is common for pieces of academic writing to have both a title and a subtitle. In these works, the title is presented first and separated from the subtitle by a colon.

For example: Nixonland: The Rise of a President and the Fracturing of a Nation

The “title: subtitle” format is rarely obligatory in academic writing, but it is prevalent, in part, because it allows for a greater range of possibilities than using a title alone.