The best popular text on exposure that I have read is "Perfect Exposure" by Michael Freeman. Although it is aimed at an audience who already understand the basics.
I couldn't bear that "Understanding Exposure" book. No idea why it's so popular. It's thin on information, the information it does give is quite shallow, and it could be condensed into a pamphlet. The author's writing style is also very annoying.
In fact, I'd say that if you want to know the very basics, one of those exposure guide pamphlets you get free with photography mags every other week will provide you with just as much useful info as "Understanding Exposure".
Once you understand the exposure triangle (pretty simple stuff) just get the Michael Freeman book.