From publisher blurb:
Knowledge Check is a product line for players and gamemasters. Each book is a collection of rules and information designed to add a little knowledge to any fantasy campaign, focusing on a particular aspect of the skill system and adding depth and breadth to your play experience.
Codes and Cyphers enters the world of secret writing, exploring the history of codes and cyphers. It introduces new spells, feats, an archetype, and even a set of codes to introduce into your game, and a band of codemakers and codebreakers to vex or help your PCs. Knowledge Check: Codes and Cyphers is the book to add a few letters and numbers of intrigue to your game!