The Oregon Byways book has a largest concentration of routes in the SW corner, not necessarily because there are more options there, but maybe because the author has more time there.
In Washington the largest concentration seems to be in the north central part. Lightly used roads last longer on the drier side of the Cascades. On the wet side, roads tend to washout in winter, or get overgrown if not maintained.
I also like to use the Backroads books by Earl Thollander. These are easiest to find used, and may be a bit dated.
Your route across Washington, I90 and US395 to US84 is the least interesting part of the state. It's a good way of getting from Spokane to Portland, but not scenic until you get into the Columbia Gorge.
