Well, interesting question. Depends on how much you know Shadowrun in some respects.
Ok, assuming you know that Shadowrun is about small groups of highly trained professional criminals and mercenaries doing jobs in a near-dystopian cyberpunk future with magic, Tolkein-esque races and dragons thrown in for good measure (and if not you do now), Emergence details a subset of magical ability known as Technomancers that are new to the 4th edition, expands on the abilities detailed in the main rulebook and throws in the rules for AI for good measure.
Most of the book, however, is fluff. But oh, what fluff it is. You can read Emergence like a novel, the story unfolds through the posts of a mixed group of Hackers and Technomancers as Technomancers and the long-secret existance of true AI is revealed to the world, the repercussions of these events and the political and economic jockeying for position by the Megacorps. It's a complex tale woven with intricacy and subtlety that links the new edition of Shadowrun and the new element of Technomancy firmly into the background of the previous editions and continues the story of the world at large. I couldn't put the book down, I read it from cover to cover in 3 hours.
In between these sections of fluff, you get rules. In the new world of Shadowrun, everything is wireless and people can be Augmented Reality (AR) enabled, meaning that you can hack anything with your trusty commlink wirelessly while your AR implants overlay illusory images of your surroundings by manipulating your brain. Advertizing pops out of the walls in full 3D as you walk through the mall and Seattle can look like an enormous emerald garden.
You have to understand that to understand how frigntening AI is as presented in this sourcebook. AI, previously secret, are mostly innately expert hackers and now they want citizenship and full rights as recognized sentient entities under law. Well, the really smart ones at least. But, like all intelligences, there are irresponsible ones along with law-abiding citizens and the rules support both. There are also less sentient AI that your hacker players can wrangle while technomancers can 'conjure' Sprites, the Technomancer equivalent of the Mage's Spirits. Lower than these, there are now Feral AI, and this is the interesting part.
The Feral AI are no better than animals but they are just as dangerous. The example given manifests as gargoyles that roam the streets of Paris in the AR architecture and are extremely territorial. They can attack the AR augmented (most people), bypass the hardware protections much like the Intrusion Countermeasures (IC) encountered by hackers and kill your brain while AR augmented witnesses watch them tear you into bloody shreds. And yeah, it takes a Hacker with combat programs to take 'em down, you can't do it with your fists.
Scary, eh?
Rules for creating these AIs are given along with samples, each with a unique personality. Along with news reports that may suggest to you plotlines or personalities for the AIs in the fluff, there are adventure outlines for you to exploit. Don't quote me on this, but I believe there's also a short section on AI PCs for those so inclined.
As I've mentioned a few times before, it contains a few easter eggs for those who know the old editions, which certainly was one of the things that made the book for me. I wasn't confident before, reading the core rulebook, exactly how Technomancers fit into the world I knew, their forebares (the Otaku) weren't as large a part of the setting. Emergence not only answered all these questions but outlined how to bring them into your game according to a timeline of news reports and local events that a canny DM could orchestrate into a series of adventures for his Runners.
Seriously, this is a great book on so many levels.
