There are several excellent bicycling guidebooks for the Puget Sound area, but only for road cycling and touring.  I haven't been able to find a standard mountain biking guide for the area, best to stick with Evergreen's Mountain Biking Trail Guide.

The 2nd edition of Biking Puget Sound: 60 Rides from Olympia to the San Juans now has 60 rides for all rider levels.  Also from Mountaineers Books, 75 Classic Rides Washington has great rides all over the state; many follow established organized ride routes. Nearly all of these are 40+ mile rides and have significant hills, but they are rated and several easier, flatter rides are included. There are also two cross-state routes.

Gene Bisbee (of Biking Bis) has a great guide for the Seattle area, Best Bike Rides Seattle: Great Recreational Rides in the Metro Area (Best Bike Rides Series).

There's also 75 Classic Rides Oregon for riders who want to head farther south.

There are several excellent bicycling guides to Washington, Oregon and the Pacific Northwest. For bicycle touring I'd recommend the Cycling Sojourner series: Cycling Sojourner: Bicycle Tours in Oregon and Cycling Sojourner: A Guide to the Best Multi-Day Tours in Washington. Both highlight a series of multi-day routes over some of the best scenery in the states. They include rides for all levels and seasons.

For a general introduction to bike touring and bikepacking, consider Basic Illustrated Bike Touring and Bikepacking (Basic Illustrated Series).

The best route maps for tours are produced by the Adventure Cycling Association. Their Washington State Parks maps have some of my favorite routes in the state.

The Mountaineers have a book available for city riding called Urban Cycling: How to Get to Work, Save Money, and Use Your Bike for City Living