The Resurrection of the Son of God, by N. T. Wright, was by far the thickest book of 2012.
It's not quite the heaviest (that would be Campbell's book), but it's the runner-up. I've read some of Wright's theology, but this is the first of his historical tomes I've opened. The implications of the book are theological, but he's approaching the topic via historical documents, using them to explore culture and ideas of the time.
Why did Christianity begin, and why did it take the shape it did? To answer this question – which any historian must face – N.T. Wright focuses on the key points: what precisely happened at Easter? What did the early Christians mean when they said that Jesus of Nazareth had been raised from the dead? What can be said today about this belief?
I found the book very scholarly--rather above my head at times, as history is not my discipline--but clearly written, and even entertaining. The most interesting part, to me, was the context that Wright places the early Christian writings in: discussions of God, death, and life after death from concurrent Greek, Roman, and Jewish texts. The detailed breakdown of such writings, their links to cultural practices, and the comparisons to early Christian writings produce a lot to think about... and also what seems to me an historically sound answer to the source of early Christian belief.