Can I use the green part of leeks for stocks? Most recipes call for the white and the inner green parts of leeks. I feel like I'm wasting half leek.

After googling a little, I've found some people consider it edible if properly cooked. I guess a couple of hours of simmering should be enough, but my book says to use only white and inner green parts for stock. Is there a reason not to use it, if properly cleaned?
Absolutely, go ahead and use it.

I always ignore the "white and light green parts" instruction anyway, and use the leek up until the point where it feels dried out instead of firm and fleshy—well into the dark green parts—and it's always delicious, even when cooked for significantly less than several hours.