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
2This doesn't explain why it is "A History," and not "The History." It explains what "A History" is in the title, but not what the OP is asking. – kiamlaluno – 2013-05-05T20:43:21.863
2@kiamlaluno Because it is a genre description, not a name. "This book is a history". There are many histories of many topics, and this is just one of them. – StoneyB on hiatus – 2013-05-05T21:07:55.657