Ohkay, ignoring half that conversation as it's not really useful to the OP's question...
To your literal question, get 6WA first. Feral Cities is ok, but really only if you're planning on doing running in Lagos or Chicago (or the other few blurbs, but they're kinda specific). Corp Enclaves could be more useful as it hits LA and Tokyo (with about the first update on Techno-chitlins you've seen in over a decade due to SoLA's demise), but 6WA would at least cover the basics until you decide where you want to go with your campaigns. People have also put in their suggestion and I'd agree with Seattle '72- as well, it's Seattle.

As to associated books to further flesh out the world- And do realize that a lot of my suggestions are about to become a long process in eBay scouring, Amazon looking, and used book store hopping unless they're already out on pdf and you prefer digital. (Not me, dead tree all the way, but YMMV)
I would put in a vote towards Corporate Guide- since it may not be to some's liking, but it does flesh out the Big Ten. I would also vote for finding
Corporate Download and
Corporate Shadowfiles (The 3rd Ed and 1st/2nd ed Corp books respectively) so you can get more layers of the onion, with at least the corps that are still around as well as any underlying causes for the others' demises that may still be around.
The three Shadows of books are a good start to basic expansion from 6WA (even if some of the stuff is dated) in the world, even different people would argue that either Asia or Europe is flawed, but I thought they were both good reads.
And then one last suggestion, Dragons of the Sixth World. It may be a bit tough, but depending on how you view the wyrms in your game it's good insight to how they can change things around.
I could go further down the line, but I think you're looking more for a general start.