The colon is conventional, indicating that what follows is the subtitle. In English books this is often a generic description, allowing the author (or publisher) to put the topic first, in bigger letters:
Hogwarts: A History
Sardanapalus: A Tragedy
Citizens: A Chronicle of the French Revolution
Middlemarch: A Study of Provincial Life
The subtitle may be an alternative title rather than a genre
Oliver Twist: The Parish Boy's Progress
Another strategy is to introduce the subtitle with or:
Leviathan, or the Matter, Forme, & Power of a Common-Wealth Ecclesiasticall and Civill
The Hobbit, or There And Back Again
Frankenstein or, The Modern Prometheus
Kurt Vonnegut famously did both:
Slaughterhouse-Five, or The Children's Crusade: A Duty-Dance With Death