Here's some further information on exposure;
Cambridge in Colour A Learning community for photographers. Sean McHugh puts a lot into his site, check it out!
For a more in depth and technical explanation on exposure, I recommend reading "The Negative" by Ansel Adams. His work has inspired folks for nearly a century and will continue to do so for as long as we admire beauty! You can get this book new from Amazon for about $14.00.
A great source for general photography, is the "Digital Photography" series by Scott Kelby. There are 3 books in the series with a fourth one coming soon. You can also get this at Amazon.
Here's a link to Amazon, just search for those books and read on. Just so you know, I'm not getting anything from Amazon for this.
I hope to see your questions and comments!