Understanding Exposure is a pretty good book. There's also tons of free internet resources out there. If you have a specific question, feel free to ask in our forums too.
You need to understand that with photography, it's all about light. You can't take a photo without any light.
Aperture controls how much light gets to your sensor.
Shutter speed controls how long the sensor is exposed to your light (so in a way, it also controls how much light your sensor is getting).
And guess what? ISO also controls how much light your sensor is getting.
Get my drift? I hope you're seeing the light.

(Sorry, couldn't resist putting that pun in!

)