The blackout has immediate consequences as cars crash and numerous people die, including a little Japanese girl. The girl's mother, Michiko, is a scientist at CERN, specifically at the Large Hadron Collider. Michiko is engaged to the main character, a Canadian scientist named Lloyd Simcoe. (Extra points for creating such a dorky Canadian name, Bob.) A third protagonist is Lloyd's partner in science, Theo Procopides.
Through internet sites and various reports, the flashforwards are determined all to have shown the same future date (some people happened to be reading the paper, or watching TV). Lloyd is a bit disturbed to find that he was in bed with a woman who was not Michiko. Theo has a more disconcerting flashforward: he had blankness. Worried that he was dead, he sets up a website asking anyone with information about his future to contact him. Sure enough, others report his death.
Ultimately, Lloyd and CERN take responsibility for the flashforward, since it happened at exactly the moment they started up the LHC. (If you'll recall, in real life, the LHC went online in 2009 and people speculated that it could create a black hole and destroy the universe. This did not happen.)
As a test of this theory, they recreate their experiment with everyone in the world seated, not driving or flying anywhere and waiting for... nothing.
But the future is not going to proceed as planned. Theo's brother, who saw himself working in the same tourist-y restaurant in Greece where he working in now, commits suicide rather than live out such a dead end life.
Here come the spoilers: eventually, the book jumps ahead to just before the moment that was flashed to. We see that Lloyd and Michiko have separated (the flash made it kind of hard for Lloyd to commit to marriage knowing it would not last). Theo is still alive and finds the man who wants him dead. Turns out the man blames Theo for the death of his wife during the blackout. They fight, they chase, Theo manages to survive.
Overall, the book posits lots of interesting theories and the premise is terrifically compelling and suspenseful. However, the characters are a bit dull and the dialogue is classic SF bland.
What you'll get in the book that you won't get in the movie: discussions of the uncertainty principle, bosons and a weird old man who wants to create immortal consciousnesses.
Links:
Sawyer talks to AMCTV about SF on television and movies.
Time Magazine video interview with Sawyer.
CERN video interview with Sawyer.
Bottom line: if you liked the show for the premise, you might like seeing how it plays out with a different group of people. If you liked the show because you liked particular characters or situations, you're unlikely to find them in the book.