I used Lhasa Wilderness yarn from Bijou Basin Ranch, which is a blend of 75% yak down and 25% bamboo. It's silky-soft, and other projects I have knit with this yarn have yet to pill. The bamboo fibers are really breathable, so it's both lightweight and cool to wear in warmer temperatures, but when it gets colder you are still nice and warm thanks to the yak down, which is super-insulating. It's the best of both worlds!
If I could knit this sweater over again, I think I would have added a few more stitches to the sleeves. Mine are a bit snug, and while I wouldn't describe them as being too small, I find I prefer a half-inch or even an inch of ease these days. Hopefully they will stretch out a bit with wear. All but one of the colors are OOAK test dyes, but you can see their current palette of hand-dyed options here.