Graphics and audio are not the only assets that we use when developing games.
Fonts are another important visual asset found in games, so don't forget to utilize fonts that are appropriate for the style of the game! Some popular web resources for fonts are:
Be sure to check the licensing for each font before using it in a game—while some fonts are released into the public domain, many are licensed and may require payment or attribution.
Don't forget that StencylForge contains a large collection of all types of game resources including fonts, sprites, backgrounds, tilesets, and audio!
The following books are not specific to Stencyl, but are references relating to the design of games:
A Theory of Fun for Game Design, Raph Koster: An interesting, visual approach to understanding what makes games fun and entertaining
The Art of Game Design: A Book of Lenses, Jesse Schell: This book breaks down each element of gameplay and examines the theory in great detail