I just finished 11/22/63 by Stephen King. The book is nearly 1,000 pages long and I read it in just a few days. I found it very very hard to put down.
It helps that the subject of the book is something that so many people are obsessed with, but what really surprised me is that the Kennedy assassination aspect of the plot was not what made the book so gripping. Instead, the meat of the book was King's spectacular recreation of the era in which the book is set, along with vividly believable and likable characters. Despite the length, the book was very tightly written and never lost its tempo, or came off the rails.