I'll second Marianne's recommendation on "Understanding Exposure". I just loaned my copy to the person who bought my Canon Rebel XT.
This link is an excellent in-depth description of how Canon's flash system works. It will explain the difference between primary flash and fill flash, and which shooting modes use each.
Once you get comfortable with the camera, the first additional accessory I'd recommend would be an external flash. If you do much indoor shooting, this will be a major step up over the built-in flash.