Light: Science & Magic
Matters of Light & Depth
Light is light. These two are excellent resources on lighting theory and they compliment each other. Definitely give these a read first so that you have a basic understanding of lighting.
www.strobist.com
Great resource for application and general speedlight tips and techniques. Just don't fall into the trap of being formulaic.
As far as your gear specifically, I do recommend getting the flash off camera, because separating flash position from camera position is a pretty fundamental thing to do with lighting. I'm pretty sure you're stuck using SC-28 or SC-29 if you want to maintain TTL functionality with your SB-600. You should be able to use an older cord if you tape off the TTL contacts though you'll be in manual. Other option for TTL is to use SB-800/900 or SU-800 as a controller for CLS. Personally I shy away from TTL metering when using flashes because results can be spotty and inconsistent, even if you start locking flash exposure.
If you'd like to go manual, you can also try radio slaves & transmitters. For the SB-600, you'll need a hotshoe adapter (since it doesn't have a sync port), but they're cheap and widely available. Radio setups themselves range in price and quality from the cheapy ebay generics to the industry standard Pocket Wizards. There are quite a few newer options within that range now too (Elinchrom Skyports, AB Cyber Syncs, etc).
Regarding the huge plethora of diffusers out there, my advice is don't get caught up in the hype. It's like a contest to see how many possible ways everyone can reinvent the stupid thing, though objectively looking at it I seriously doubt you'll be able to see any critical differences in actual usage. Gearheads can scrutinize these things to death in side by side comparisons, but ultimately a little piece of plastic isn't going to affect your lighting anywhere near as much as placement, coverage, technique, exposure balance, etc.
In short, start with those books first. Learn how to see light and work it. Don't worry so much about the gear.