The Screwtape Letters: Letters from a Senior to a Junior Devil - CS Lewis
A delightful philosophical, moral and social analysis that still resonates sopowerfully in 2012 that it confounds the fantasy of human progress.
Even though a monologue it evoked comparisons to the style of socratic dialogues, or even Thus Spake Zarathustra.
The content echoed Machiavelli in presenting a detailed manual for the manipulation of weak humans to the ends of others. Each letter skewered some social habit or tendency that drifted into evil.
The overwhelmingly common and mundane nature of evil (i.e. doing deliberate harm) is instructive in a world where the word is rarely used at all even for the most vile of human acts.
Still stands as a wonderfully effective moral primer after so many decades.