[akullpp]

The New Testament, or any book that explains the key themes. Even if you are not religious, Western societies are built around this. If you live in the West or do business with the West, it explains many explicit or implicit cultural norms.

If you want to go deeper in understanding the West, it's recommendable to also get a rough idea of it's philosophers, especially the Greeks and the important ones up to Kant, including Descartes' idea of the social contract.

If you still want to know more, you should read about the most important economic ideas, e.g. Smith, Keynes, Marx.

[marginalia_nu]

I'd toss in St Augustine as well, at least Confessions. You can't properly understand western thought without understanding the catholic church, and you can't understand the church from the bible alone.

Maybe Cicero too, like De Officiis at least (although De Natura Deorum is a more enjoyable read imo). Cicero was a second rate philosopher and incredibly full of himself, but he was extremely influential for a very long time.