As a social studies teacher, I like to tell people that I know everything... just not all at once. :)
The fact is, while I can't know everything, I have complied a list of resources so I can get an overview of a vast variety of subjects rather easily. Today, I found a new resource:
From the desk of Dr. Oskar Seyffert. this 1895 illustrated dictionary runs from Abacus to Zosimus. I'm sure it's horribly dated, but you have to love a dictionary that ends with a Greek historian and an Index. The first seems perfect to me, but the second is most definitively wrong in my internet addled brain.
Archive.org has 10 different files and 19 different ways to download this book for free. Check it out, it could be a great campaign helper for you game.