I think the best online resource is the Strobist (and it's free):
http://strobist.blogspot.com/2006/03/lighting-101.html
And this is a great book covering photographic lighting in general: "Light Science & Magic, An Introduction to Photographic Lighting"
I would agree, that these are some of the best resources, to understand what is going on. Some people just want "recipes" or proven setups that they can follow - they would need different resources from these. I learned the huge majority of what I know about lighting long before these sources, but if the "me" from today could go back in time to educate the "me" of then, these are two of the resource I'd want to send back.
When you understand more about the sources and controlling them (via diffusers and reflecting, etc) per "...Science & Magic" there is so much that you can do. Even if it is just a small boost to overcome slight problems with existing light. Using a gel on the flash to match color, and then a low-power bounce flash to accent the scene can really fine tune a scene (but I don't think either of these resources covers much about balancing with gels).
I made a full-time living in photography starting from about 1970s, including a lot of portraits studio type work and photofinishing, including lighting system design work. Long ago I became convinced that many people who were adamant about sticking strictly to ambient lighting really didn't understand how to put flash to work. So you stick to what you know.
Personally, for just casual indoor shooting (of people) with available light, I like to gel a pop-up flash, then set it at -1 to -2 exposure compensation. It helps overcome "racoon eyes" from overhead lights, yet almost no one realizes it's there when looking at photos. (As a note, for people who don't have any gels, a piece of clear C-41film (orange base) taped over the flash comes pretty close to incandescent lighting.)