West to the Sun — Noel M. Loomis

It stretched out there to the west -- the endless, silent, savage wilderness only a very brave man or a fool would challenge. Dan Shankle was no fool. His mission made him desperately brave. For somewhere in these killing plains lay hidden the contrabandista, a man who traded guns to the Indians who used them to massacre a bloody swath across the frontier. He had to be stopped. Shankle rode West to the sun, into bitter country that gave no quarter. It was kill or be killed -- by a white man, an Indian, or the brutal savage land itself.