My new long-term project is Gibbon's Decline and Fall of the Roman Empire. And it definitely is long-term. The penguin edition of vol. 1 (of 3) weighs in at 1000 pages, with a 100 pp introduction.
At about 200 pages in, I'm enjoying it. Gibbon is very detail-oriented, and so there are some good stories in there. In particular, I was really surprised when, after covering the emperor Pertinax in what seemed like great detail, Gibbon then relates that Pertinax only ruled for a couple of months.
But the story of Pertinax is important for Gibbon, because it shows the extent of the corruption of the Praetorian Guard, whom he blames for the first step down the path of decline.