What are some novels that display the theme of power corruption?

As controversial as it will be, Atlas Shrugged demonstrates corruption of power pretty well.

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Warlock by Oakley Hall explores how money corrupts those who have it and how power corrupts those appointed by the gentry to protect their interests.

Cornelius Fudge in Harry Potter and the Order of the Phoenix springs to mind.

Not sure if you can use this, but it's a fairly major theme in Star Wars, though it's more a theme of the universe itself than of the movies or any individual story within the universe. The entire reason for the Jedi order existing is to reign in the corrupting influence of the Force. They train children from birth to be subservient and to deny their emotions so they can be powerful without being corrupted. It's why, despite having access to knowledge and understanding the Senate could never comprehend, they still obey the Senate. They make a point not to seek power they don't need, because it is a corrupting influence they don't want; rather, they take it up with a heavy heart when the galaxy needs them to.

The Sith, on the opposite side, seek power religiously in their quest for victory and freedom, and are literally corrupted by their power, physically deformed by the raw energy of the Force coursing through them. For a more traditional interpretation of the word though, they also lose all empathy toward other beings and essentially come to refuse to be subservient to anything, to the point that Sith traditionally always kill their masters for their power, and to the point of Palpatine orchestrating a war that killed more people than actually exist in real life just so he could be named Emperor. The entire Sith order and Palpatine in particular essentially define the phrase "power corrupts."

Apologies if you can't use movies; can't think of anything else that hasn't been posted. If not you may be able to throw something about it in anecdotally to demonstrate the theme. Can always use more page-filler. Or maybe read Darth Plagueis and use that as one of your novels.

• Casual Vacancy by the lovely J.K. Rowling (political satire)
• Ender's Game by Orson Scott Card (Demosthenes and Locke would surprise you)
• Mistborn Trilogy by Brandon Sanderson (fantasy but with heavy themes of corruption)

Please post book recommendation requests in r/SuggestMeABook or in the Weekly Recommendation Thread.