Hhmmmm - most people have trouble the other way around - finding info on the Danish and Norwegian guns!  :-)

There are three books available on the US Krag. 

(1) The U.S. Krag Rifle and Carbine - Poyer & Riesch - North Cape Publications

(2) The Krag Rifle Story - Mallory & Olson

(3) The Krag Rifle - Brophy

(1) is in print and available for about $25 - it will answer a LOT of questions. (2)(3) are out of print, but copies show up with some regularity, in the $75+ range, if you decide to search further.