The Seattle Sourcebook is a gamemasters reference for Shadowrun. It furnishes all the information you might need to run a campaign in Seattle.
The major part of the book is an overview of all the districts that make up the Seattle Metroplex, with histories, geographical descriptions, demographics, and so on given for each district as well as Seattle as a whole. The most important part of each district chapter consists of short descriptions of various buildings and locations. The last (shorter) chapters talk about how the 'plex is governed, organized and non-organized crime, and other things that are handy to know.