Loc: Portland, OR
Most WA guidebooks divide the state into regions and cover just that region in greater detail. This is a good approach, since WA has such a wealth of trails that a book that covers the whole state either has to focus on some narrow category, such as loop hikes over 20 miles long, or else it would barely scratch the surface.
The WA guidebooks published by Falcon tend to be good, but a lot depends on how thoroughly the author has done his homework. I know the Falcon guides to the North Cascades and to Goat Rocks are both good ones.
The Mountaineers of Seattle publish a number of guidebooks, too. They are fairly uniform in quality and if you like one, you will probably like them all. They often pad out their trail descriptions with complaints about how this or that trail runs through an area that deserves wilderness protection, but lacks it, and while I tend to agree, this gets old after a while.
A book that covers all WA and I think is good is Backpacking Washington, by Douglas Lorain. It concentrates on multi-day routes you can take, with many side trips mentioned, too.