Old encyclopedias are gold mines for seeing how society has changed. In 1950s encyclopedias, Christmas is listed as a pagan ritual. In modern entries, it is listed as a Christian holiday. I first became aware of this in the 90s, so in 40 years, a group rose to power enough to get encyclopedias to update as they saw fit. So, that in itself is information gleaned from an encyclopedia that wasn't actually written in the pages.