What is the latest iteration of the Seattle Source book? Also Besides the Sixth World Almanac what book gives the best over view of the current time? What is the most worth while setting book released for 5th?

I think the most recent book on Seattle is Seattle 2072 for 4th edition. The best timeline? It's not a book but check out the Neo-Anarchist Podcast. There haven't been a ton of actual setting books in the vein of Seattle 2072. You have lockdown which has a bunch of stuff about Seattle but I wouldn't run there at the moment. All of the shadows in focus PDFs are good but they aren't actual books. Hard Targets was just released (haven't picked it up, waiting for the dead tree) and I've heard it's write up on Havanah is really good.

Isn't Lockdown about Boston, not Seattle?
Anyway, yeah, the most recent Seattle book is Seattle 2072 with probably the most serious change between it and the current time line being that the Ork Underground is now an official district just like Bellevue or Renton or Redmond, with its own cops and officials. This was the result of the last set of Seattle based Missions adventures for 4th ed.

There is a Seattle boxed set in the pipeline, just FYI. It won't help answer this question, but there is some Seattle love coming.

Woops, I derped. It should have said Boston, guess Seattle was on my mind.