Backpacking Washington, by Doug Lorain, 2007 edition. There is also a series of guidebooks from the Mountaineers, but most of those are day hikes. With the aid of maps, you can, of course, put a bunch of day hikes together.
Note, though, that guidebooks are already out-of-date by the time they get to the bookstore shelves. It's vital to get the latest information on current conditions. It's also a good idea to have a "Plan B" in case of last-minute fire, flood, etc. That happened to me in Wyoming in 2008.
There is also a lot of online info, often with lots of photos--here are some sites:
The aforementioned Seattle-area hikers' website: http://www.nwhikers.net/
Washington Trails Association
For southwest Washington info:
This site has a Field Guide with lots of good info.