I'm trying to buy a book by Aristotle, and a book by plato. Which ones should i buy?

For Plato try the book titles great dialogues of plato,for aristotle no idea

Like pinduck says go to any university,but look up Brittanicas Great books of the Western World, volumes 7 for Plato,and volumes 8,and 9 for Aristotle.

Any one of these books by Aristotle:

Physics

Metaphysics

Nichomachean Ethics

Politics

though I have to say, I particularly enjoyed Metaphysics and Nichomachean Ethics.

As for Plato:

The Apology

Crito

Phaedo

The Meno

Gorgias

The Sophist

The Symposium

Parmenides

Phaedrus

Protagoras

I love anything of Plato (though I have a soft spot for The Meno) so I can't narrow it down anymore, I would honestly read them all at some point =)

Happy Reading!

Aristotle, The Nicomachean Ethics.

Try going to a local university library (even if you're not a student you can still get in) and locate various books by these guys. Then, at your leisure, you can peruse them and compare, and make your decisions that way.