Several historical maps are found in Ralph N. Preston, Early Washington: Overland Stage Routes, Old Military Roads, Indian Battle Grounds, Old Forts, Old Gold Mines (Corvallis, Oregon: Western Guide Publishers, 1974; FHL book Q 979.7 E3e).
The Family History Library has several maps of Washington. Larger collections of maps are available at the University of Washington in Seattle and Western Washington State College in Bellingham.
University of Washington in Seattle: http://www.washington.edu/
Western Washington State College in Bellingham: http://www.wwu.edu/