[tjic]

The Oxford English Dictionary.

It defines every English word they're going to ever come across outside of technical literature, it's got tons of historical examples, shows the connections between English and other languages, and more.

If we extrapolate the speed with which Google is learning to harvest latent information out of data dumps out just 20 or 30 years, I wouldn't be surprised if aliens smart enough to get here could pull a huge amount of culture and history out of the OED.