I just read a small commentary on history books from Thomas E. Ricks on Foreign Policy, who gave an insightful reason why the first half of books are often better than the second.
Listen to what he wrote:
The writing of most books, I think, is a race between resources and understanding. As the author proceeds, his or her resources (energy, time, money) diminish, even as his or her understanding of the subject increases. The question is maintaining the resources long enough to finish at least a first draft. In many and perhaps most cases, fatigue overcomes the author, and so many books are weaker in their endings than in their beginnings. That is the way books end, not with a bang but a whimper.
Read the whole thing here. It’s short, but packs a punch.