Websites:

Investopedia Dictionary entry for Risk Tolerance
Basic definition enhanced with links to Related Terms and related links.

Personal Finance: Investment Risk Tolerance Quiz
A quiz to get an idea of your investing risk tolerance

Risk in Basic Investing
Illustrates the risk/rate-of-return tradeoff and challenges you to think about your personal risk tolerance

Morningstar's Help & Education
Click on View Complete Course Catalog under Investment Strategies, click on Portfolio Tab, click on Course number 103: How Much Risk Can you Tolerate?

Books:

Investing for Dummies by Eric Tyson
332.6 TYSON 2006