<p>I read Wittgenstein’s Tractatus Logico Philosophicus a little while back; it was fairly short, and its difficulty was probably on par with most of the other stuff I’ve gone through.</p>
<p>The Prince and The Communist Manifesto, while not being particularly difficult, are also a fairly short works (and could fulfill your arbitrary political philosophy quota!). Other books that would fit here would be various federalist papers (get an anthology) and Paine’s many pamphlets, etc, which are all quite readable. Now that I think about it, few phil. works are published in giant works; they’re usually concise and self contained.</p>
<p>Kant’s 3 critiques are also short while maintaining a fair level of difficulty. Ecce Homo (Nietzsche) is also short/difficult if you haven’t read the works he self-refers to.</p>
<p>Edit: If you’re just getting into philosophy I suggest that you’d start at the beginning and from there trudge forward. Pick up a copy of the Republic, Nicomachean Ethics, and Plato’s various other Socratic dialogues and work from there. You could also just subvert the entire “primary work” angle and read interpretations and whatnot that might paint it all in an easier to understand light. Try out [Stanford</a> Encyclopedia of Philosophy](<a href=“http://plato.stanford.edu/]Stanford”>http://plato.stanford.edu/) for some easy to access knowledge, or even wiki it all (what I did when first starting out 
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