Understanding Exposure

It's my big struggle with digital SLR photography: just how exactly do aperture, shutter speed, and ISO relate?  If you change one, what do you do to the others?  And what settings get you what type of photo?  It finally sank in that long shutter speed = veiled waterfalls, and that large apertures (small f number) = blurry backgrounds.  Those were a couple of key things I wanted to be able to achieve with my camera, but I've been craving more.  I can deal with start trails now, even though the results aren't yet what I'd like them to be, but ISO is still a mystery.  I never changed it.  I needed someone to explain it to me in simple terms, with examples to illustrate it.  A friend recommended Bryan Peterson's Understanding Exposure to help me out.  I found it in the library catalogue, placed a hold, and after two months of waiting (it's a popular book), I have it.  I'm still on the first chapter, but things are starting to sink in.  I'm starting to be convinced that more manual can mean improved images, or at the very least, much more artistic control.  I've been taking pictures of the exact same things, but with different settings.  Change ISO and shutter speed, stop one up and stop one down and you'll get the same result.  That's cool stuff.  Now I'm on white balance and how certain settings can bring out certain colours.  I'll absorb what it can teach me, but the book most certainly is a hit.  Photography enthusiasts who've read it say it's one of the best books they've seen on the topic, and so many say that it finally clarifies things.  Here's to figuring it out, and lots more trial and error photos to come.