The question of roof pitch ultimately depends on what you want to grow. If you want to grow during the winter a steep roof pitch helps, if you want to ripen fruits and veggies in the summer you will want a more shallow roof pitch. If you want the best sun in the summer (when the sun is at it's best) then a 3:12 roof pitch
should be about right for your latitude.
On the subject of the New Alchemy Institute, the Arch is a great place to go for ideas as well. They've built a number of bioshelters in the 70's and 80's.
It will take a second but here is their link for the
Bio-Shelters Primer. They have lots of good publications available on their
website.