Greg Green, Marijuana Grow Bible, for sure. Also my experience, after testing identical clones from the same plant in two exactly identical veg cabs, one running 24/0 and one running 18/6. The one running 18/6 was about a week of growth behind the 24/0 box after one month (which makes sense, since it lost about a week's worth of access to light energy over that time).

The main reason why some people run 18/6 is because their ventilation system isn't adequate to control the temps if the lights are running 24/0. And if you can't control the temps, you risk hermies. But if you can control the temps, and have no other reason to slow down growth, then why would you? I've never found a reasonable answer to that question, personally, so once I'd done the experiment and seen what I needed to see with my own two eyes, that was enough for me and I've been 24/0 ever since.