Hello nerdcore,
Depending on what you're looking for, I'd strongly recommend:
Seattle 2072 - great fun read and very useful if you plan on playing in Seattle.
Attitude - again a great read and a really good source book for giving little details about what the 2070s are like - music, sports, entertainment etc.
The Sixth World Almanac is supposed to be a kind of an introduction to the setting and it kinda is, but it does paint the world and its history in VERY broad strokes. I enjoyed it a lot, but I'd definitely recommend reading the history stuff at the beginning of the core rulebook before reading it.
If you're going to be GMing, I'd also mention the Shadowrun Missions series, which you can get for $4 in pdf. Te current season (season 4) is set in Seattle and each mission is very simply laid out with all the stats you could need, colour pictures of all NPCs and lots of ideas for tweaking/extending each scene. They're ideal for a new/busy GM (like me). Incredible value.
You've already made a great purchase in the form of the Runner's Toolkit, which has loads of great stuff. The Anatomy of a Shadowrun is especially useful.
Hope that's of some help.