The term "principle fiend" is not a standard literary term and doesn't have a clear meaning within the context of a book. It's likely a misunderstanding or a creative use of language.

Here's a breakdown of why the term is unclear and possible interpretations:

* "Fiend": This usually refers to a malevolent being, often demonic or evil.

* "Principle": This could be interpreted in a few ways:

* Core belief: The main idea or philosophy that drives the story.

* Character's motivation: The primary reason a character acts in a certain way.

* Moral principle: A guiding principle that governs the story's ethics or themes.

Possible Interpretations:

* An Evil Force: The term "principle fiend" could be used metaphorically to describe a powerful negative force in the story, like an evil ideology or a corrupting influence. This would be akin to calling something the "principle of evil."

* A Motivating Force: It could refer to a character's deeply held belief that drives them to commit evil acts, essentially their "principle" of being a "fiend."

* A Story's Driving Conflict: The "principle fiend" could represent the overarching conflict of the story, acting as the antagonist or driving the narrative's tension.

In conclusion, without more context, it's difficult to give a precise answer. However, it's likely a creative phrase referring to a significant negative element within the book. It could be helpful to provide more context about the book or the specific usage of the term for a more accurate interpretation.