This was the best thing I've read in a long time, but I'm suprised that nobody mentioned a guy I consider to be one of the very best at landing a left hook- that being Bob Foster. The left hook is my favorite punch and I think he was able to camouflage it and deliever it in more ways than anybody else. He didn't just hammer away with it.
Personally I think there is too much talk these days of long, leaping and lead left hooks. To me, it is primarily a short countering blow. In leading with a long hook you commit the same sin as when being less than judicious in leading with the right: you leave yourself far too open. These are, essentially, set punches, meaning that you plant your feet in order to get your weight behind a hard punch. Thus it is best to have your opponent coming forward when you throw them. To make this happen even when you are the 'aggressor' is the whole point of boxing.
Circle right, then hook. Not many do it these days and you avoid the right hand. Beware his hook but, since you should know it is coming, you should be able to avoid it and counter it.