A Checklist of Washington Post Offices 1850-1989 by Richard W. Helbock is a compact post office guide with scarcity ratings. Washington has for some years been one of the best states. It is a large state with many post offices and a fairly good range of available material.
More collectors collect the entire state than focus on specific areas.
Whatcom county is the one areas with especially strong demand. However, Seattle and its environs are avoided by most of the collectors. Territorial material is in the most demand and is quite scarce after the half dozen largest cities. There are signs that the bull market in Washington covers has slowed. Fewer people are starting collections.