The first book everyone should read, or in my opinion, own, is Mountaineering: Freedom of the Hills: 50th Anniversary. This is the bible of outdoor adventuring and the 50th Anniversary edition is a very nice edition. A lot of the material is related to climbing, but I find it useful as a hiker.
The 2013 book, Trail Tested: A Thru-Hiker's Guide to Ultralight Hiking and Backpacking, is a less-technical, but very thorough guide to different hiking styles, gear, and other backpacking information.
Campcraft and Navigation
Again, the Mountaineer Books are the gold standard - Wilderness Navigation: Finding Your Way Using Map, Compass, Altimeter & Gps (Mountaineers Outdoor Basics) is a slim guide to all basic navigation skills. Supplement with Wilderness Basics: The Complete Handbook for Hikers & Backpackers.Wilderness First Aid, Third Edition: Emergency Care for Remote Locations came with my Wilderness First Aid class materials, and I've found it to be a good reference.
For ultralight hikers, or people that just want to reduce pack weight, Ultralight Backpackin' Tips: 153 Amazing & Inexpensive Tips for Extremely Lightweight Camping is excellent, one of my favorites, loads of valuable campcraft information. There are also two additional books in this loose series, again filled with handy tips and tricks that even seasoned backpackers might not be familiar with: Lighten Up!: A Complete Handbook for Light and Ultralight Backpacking (Falcon Guide) and Allen & Mike's Really Cool Backpackin' Book: Traveling & camping skills for a wilderness environment (Allen & Mike's Series). An additional source of lightweight camping information is the 2005 bookLightweight Backpacking and Camping: A Field Guide to Wilderness Equipment, Technique, and Style (Backpacking Light).
For those who climb some of the non-technical mountains:
Andrew Skurka's 2012 book,The Ultimate Hiker's Gear Guide: Tools and Techniques to Hit the Trail, lives up to its name. It's currently the definitive guide to selecting outdoor equipment for all conditions.
For I-90 day hikes within an hour or two of Seattle, this is all you need: Day Hiking: Snoqualmie Region (Done in a Day)
For those venturing a little farther, to the Glacier Peak Wilderness and vicinity, consider: 100 Hikes in Washington's Glacier Peak Region: The North Cascades. Even older editions of this classic are quite useful.
Best Loop Hikes Washington (Best Hikes) has a one hundred loops within Washington State.
For information on the Wonderland Trail: Discovering the Wonders of the Wonderland Trail: Encircling Mount Rainier. I have not reviewed One Best Hike: Mount Rainier's Wonderland Trail , but it should add valuable information.
Update 9/5/12: There's a new book out about the Wonderland Trail - Hiking the Wonderland Trail: The Complete Guide to Mount Rainier's Premier Trail - and while I haven't read it, the reviews have been quite positive. Plus, since it just came out, it represents the 'freshest' information available in book form.
If you want to hike in Eastern Washington, the recently released (2013) guidebook - Day Hiking: Eastern Washington Kettles-Selkirks Columbia Plateau Blue Mountains - has the latest
Hiking with Dogs
The primary hiking guide - Best Hikes With Dogs in Western Washington: Western Washington - is my favorite, but this one - The Dog Lover's Companion to the Pacific Northwest: The Inside Scoop on Where to Take Your Dog (Dog Lover's Companion Guides) - covers the entire NW (Oregon, Washington, British Columbia). Both are good investments for the those who take their dogs hiking.
National Park Guidebooks
Focusing more on the terrain and geology rather than the park proper, The North Cascades Highway: A Roadside Guide is a great companion to any road trip through the park.
It's hard to find good places to hike in the winter, but Winter Hikes: In Puget Sound and the Olympic Foothills lists 60 of them. Best Rain Shadow Hikes: Western Washington (Best Hikes) has 50 more that are likely to be dry in the winter.