The NW is such a varied place that there are really many histories. Anything you can find about the Lewis and Clark Expedition and the later Oregon trail are central.
The coast is different. Astoria OR was named after Jacob Astor, whose fur trading company established Fort Astoria, at the mouth of the Columbia. Seattle is situated on maybe the most favored west coast port site north of the SF bay, it had to be big. But a huge boost was given by the Klondike gold rush, and later from the founding of the Boeing Co. and WW2.
Another giant piece of NW history is the history of the native tribes. The fishing tribes along the coast and along the major rivers had a unique North American culture, and a high material standard of living. Of course the USArmy, General Custer, Chief Joseph and the eventual creation of the reservation system is very important.
Another big hunk of history is created by the railroads pushing through the Rockies in Montana and Idaho, and again the Cascades.
Don't neglect the silver mines in Kellogg and Wallace, and the rough social scene that was part of that.
The great thing about all this is that once you are here, it can become a lifelong habit to visit the sites, the great Indian museums, and the dams- the Grand Coulee Dam was a magnificent achievment and is a magnificent sight today.
Add in some study of the Portugese explorers into the Straits, fishermen from everywhere, and our colossal logging industry and you will have a good beginning.
Erna Gunther, who I met, wrote The Ethnobobany of Western Washington.
Erna Gunther and the Ethnography of Western Washington
She got her information from Indians still following the old ways. When my wife and I lived on the coast we lived very close to this way, this book explains a lot about the wet side.
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