Strobist is pretty much the end all, be all guide to flash photography. Seriously. It's like 2 complete interactive college courses, laid out in an easy to understand format. Oh yeah, and it's FREE.
As far as the whole ttl vs manual discussion, I use manual 90% of the time. If the flash is on camera it's almost always on ttl, until ttl doesn't give me what I want and then I'll step into manual. But I almost NEVER use on camera flash, unless it's a true run and gun situation. The only time I use ttl off camera, would be when the flash to subject distance isn't consistent. The best example of this would be shooting sports, predominantly basketball. I tried shooting basketball on manual, and it's hard because it only gives you a small area on the court where your exposure will be correct, so TTL is huge for basketball. In a larger gym where I could get the flashes farther from the court, I might be able to get away with manual (due to the inverse square law, the closer you are to the flash, the faster the light falls off) but I haven't found a local gym large enough to try it.