I own each of these books for hiking around the Seattle Area. Some of the books do overlap with others on popular hikes, but all the books offer their own view and thoughts on the hikes.
60 Hikes Within 60 Miles: Seattle: Including Bellevue, Everett, and Tacoma my current go-to book for local hiking on new trails. Its second edition was updated this year (August 18, 2009). The trail maps and route info are very well done -- plus it offers a number of Eastside hikes often not found in local hiking books.
Take a Hike Seattle: Hikes Within Two Hours of the City is my second favorite book for finding good hikes around Seattle. It was one of the first books to have a good map -plus- route information. Since it was published its starting to get a bit out of date - example the Tiger Mountain Big Loop hike on page 56 had a route that is under some redevelopment work. Otherwise, this book has great information about hikes around Seattle.
Beyond Mount Si: The Best Hikes Within 85 Miles of Seattle is a true Hikers book with 70 hikes included in this one small book. The book has the basics: Very good maps, "how to get there" info and some turn by turn directions. The trek descriptions are short -- but very focused on what's important. Its a great book, but starting to get a little out of date -- as it was printed in 2004. Hopefully, a second edition is coming soon.
Day Hike! Central Cascades: The Best Trails You Can Hike in a Day is one of those books that could have been much better. Its got all the great hikes, nice layout, plenty of the basics covered -- but the Maps are pretty disappointing. I am sure it was a good intent, but the maps in this book look like photocopies of b&w topo maps with the hike route drawn on top. Its hard to explain, but the end result is a not so easy to read/view map on many of the hikes. Plus some of the photos are just poor. My copy was from 2005, maybe the newest edition is better.
55 Hikes Around Snoqualmie Pass: Mountains to Sound Greenway was written by some Seattle hiking pioneers Harvey Manning and Ira Spring (photographer). This book was written in 2001 so it has some outdated information. It is a good book but often focuses more on trail planning in the area with a number of "proposed" trails.