I haven't flown through 1,000-plus pages so fast before... that's not necessarily an endorsement, just a fact...
As one might expect, there is certainly an otherworldly element to "Under the Dome," in which an invisible, impenetrable shield suddenly descends on the small Maine town of Chester's Mill. But what makes it an excellent read - and indeed, what contributes to its true horror - is its study of the people in this small town.
King gave a lot of credit to his researcher in the afterword, and it shows. In addition to a fair amount of technical details regarding medical practice and hospital procedure, there is the matter of the Dome itself. While most readers are busy thinking about where exactly this dome came from, King slowly begins to ratchet up the tension as its environmental effects start to become clearer: pollen begins sticking to it, muddying the residents' view of the outside world... there's no electricity, so everyone is running off of propane-powered generators... the temperature is slowly starting to rise... there's no breeze, and the air is starting to stink.
And no one can get out.
The situation is primed for panic, but most of the town seems to be taking it remarkably well. Enter one of the most unique villains in the King pantheon: local used-car salesman and Second Selectman for the town council, Big Jim Rennie. A smug, self-important man, Rennie never swears, and is a faithful worshiper at the local holy-roller born-again church. He has cheery, mildly-annoying commercials on the local radio station ("You'll be wheelin' when Big Jim is DEALIN'!"). But he also has a lust for power and a dirty secret to protect, and the Dome presents him with the opportunity to indulge both vices.
Not surprisingly, the Army is interested in the dome. They're trying to get in touch with an ex-military man who's stuck inside: Dale Barbara, who's currently working as a short-order cook in the local restaurant; unfortunately, "Barbie" had the occasion to make Big Jim's shit list before the Dome came down. So when the Army's plans don't jive with Big Jim's - and with the Dome looking more and more permanent - things devolve fast. Now, that's a tricky claim to make for a book that runs nearly 1,100 pages, but short, snapshot chapters keep things moving at a brisk pace.
King explores themes of alienation (how different people in town handle being trapped under the Dome), environmentalism (the Dome's rapid pollution as a metaphor for global warming) and morality (what people under pressure will do to one another when there are no legal consequences).
The book's climax doesn't have the emotional impact of the buildup - King paints a desperately frightening portrait of desolation as conditions in the Dome reach their worst - but it's well worth your time.