As Crunch says, the extended core isn't out yet. However, as far as 'flavor text' to bridge the gap between 2nd and 5th, there are a few books that will help you make the transition.
Sixth World Almanac - the first half of the book is the 'history of the world' up to 2072.
Renraku Arcology: Shutdown, Brainscan, System Failure - These three campaigns are useless, crunchwise, but they formed one of the core plot arcs of 3E, leading up to when the whole bloody world went spiraling down the drain.
Emergence - A 4E campaign. Likewise, useless in terms of crunch, but it does give a good idea of how the world collectively ran screaming when technomancers (latter-day Otaku) and AIs became public knowledge, instead of being urban legends in the runner community.
Street Magic - The magic sourcebook for 4E. While the crunch will change somewhat between it and the 5e book, there fluff hasn't changed, for the most part, so stuff like 'what it is like to be a mage in the 6th world' is going to be the same.
Augmentation - Nanoware exploded and changed the game field completely, so the crunch here is all worthless. However, some of the Cyberware, Bioware, and Geneware augmentations could be converted without too much trouble. Also, the chapter on what medical treatment is like in the 6th World is still invaluable until new info comes out.
Vice - Pure fluff. Gives you an in-depth look at the various syndicates, and some of the larger gangs. This won't have changed that much.
Attitude - Another piece of pure fluff, much like the old Shadowbeat book, apparently. Gives you more of a 'day in the life' kind of feel. Still perfectly good.
Corporate Guide - An update on all the megacorps. Some of this has already changed, but it still gives you a more up to date look than Corporate Shadowfiles does.
Running Wild - Mostly fluff, dealing with all the wild critters and nasties in the world. A lot of this stuff won't change much, except for the things with the Infected and Dragons.
Aztlan, Tir Tairngir, The Land of Promise, Shadows of North America, Runner Havens, Corporate Enclaves, Feral Cities, Seattle 2072, Twilight Horizon, Rotten Apple: Manhattan (PDF only), Montreal 2074 - Setting books. The basics don't change too much over time, and even if they do change somewhat, there are a lot of place names, 'who's who' and more that you can still use. Aztlan is the oldest one in the batch, I believe, but it is still the best (only) source on the Azzies I've seen, much like we haven't had a real update on the Carib League since Cyberpirates.
WAR!, Spy Games, Conspiracy Theories, Dirty Tricks, The Clutch of Dragons, Storm Front - In addition to providing useful setting material for Bogota, Denver, DeeCee, London, and the CAS, it also sums up the major plot arcs of 4th edition, up to the transition to 5th.
