HARNPLAYER: A PLAYER'S GUIDE TO HARNWORLD (1994)

From the Introduction:

HârnPlayer is comprised of three parts. The first part, Hârnview, is an overview of the geography, politics, economics, law, religion, and history of Hârn. The second part is a Book of Common Knowledge, which covers the Hârnic legends and folklore known to most folk. The third part is a players' Hârnic Dictionary,l a comprehensive alphabetical reference which players can use to look things up on an ongoing basis.

The information provided is more than a typical player probably would know, but nothing that one should not know. No matter how much reading is done, players will still be ignorant of much that a medieval person would take for granted. A bit of extra knowledge of the world compensates in a small way and makes the game more interesting.