If you've been gone since 2E, the books I'd recommend getting are Seattle 2072, Sixth World Almanac, and Storm Front, for sure. There are other books which are heavy on the fluff (which, as was said earlier, is editionless) that give you a more in depth view of the metaplot as it advanced. Here's a list of the books since 2E I'd recommend you grab.
3rd Edition:
Loose Alliances - Pretty much 100% fluff, describing a lot of the little (and not so little) organizations around the world, in a bunch of different fields.
State of the Art 2063 & 2064 - Good 'atmosphere' pieces, and some of the fluff is definitely still relevant in 2075.
Renraku Arcology: Shutdown, Brainscan, and System Failure - These three campaigns are mostly useless unless you want to go back and do a 2060s game, but they give you the best blow-by-blow of the events that brought the world crashing down, and led into 4th edition.
Others - There's a bunch of older books that can do some good worldbuilding for you. Lone Star, Tir Tairngir, Aztlan, Shadowbeat, Threats 1 & 2, Corporate Shadowfiles, and so on.
4th Edition:
Emergence - Deals with the emergence of Technomancers and AIs, and how the whole world went fraggin' INSANE. The campaign stuff is useless now, but there's quality fluff that gives you things as they happened, and unless you're playing a kid barely out of diapers, then this is something your character would have lived through.
Ghost Cartels - While not as globe shaking as the Emergence, the Tempo plot rocked the world like wars between criminal syndicates all over the world are wont to do.
Vice - An in depth look at modern syndicates, giving a look at their organization and culture. Very helpful if you're playing someone with ties to a syndicate or major gang.
Street Legends, Street Legends Supplemental, 10 Mercs, 10 Jackpointers, 10 Gangs - All 100% pure fluff (minus a few statblocks). You've got bios on even the likes of Harlequin, Hestaby, and Puck, information on the Free Marine Corps, and more.
Running Wild - Mostly fluff, and there isn't really a better supplement for dealing with nonhuman sapients, spirits, the infected, and AIs.
Feral Cities, Corporate Enclaves, Runner Havens, Manhattan: The Rotten Apple, Montreal 2074 - Setting books. Like Seattle 2072, these are priceless for fleshing out your sprawl.
Corporate Guide - An update of info on the megas, also includes fluff about what life as a wageslave is like, and what it is like growing up in mother corp. Some of this has changed, but most of the fluff is still solid.
WAR! - If you remember the old Fields of Fire supplement, this is an excellent book for advancing the idea of what a merc's life is like, if you want to blur the lines between runners and mercs a bit. The crunch is going to be out of date, but the fluff is solid, and there's also details on the beginning of the Aztlan-Amazonia war.
State of the Art 2073 - Like the old SOTA supplements, this has a wide variety of interesting things.
Spy Games - Shadowrunners and spies have a lot in common, but there's a big difference between the two. Lots of good fluff here about different spy agencies, as well as giving you an idea of what James Bond is like in the 6th World.
Conspiracy Theories - A run down on different conspiracies, from the scary to the crazy. Also talks about how to use (or abuse) conspiracy theorists for fun and profit.
Artifacts Unbound - Gives a solid wrap-up of the Artifacts campaign, as well as the craziness that happened in Season 4 Missions with artifacts.
Dirty Tricks - Politics. Need I say more? Also wraps up the Prop 23 side of Season 4 Missions.
Clutch of Dragons - Wraps up and advances all the dragon and immortal elf plots, including the Dragon Civil War.
Attitude - Like the old Shadowbeat, this is almost 100% fluff, and is all about the day to day life of people in the 6th world. Great read, and very useful.