Ken Follett - World Without End.
Didn't think I'd like the predecessor, Pillars of the Earth, as it took a long time to discover how all his various separate plots and people would come together but once it does, it really keeps your interest.
This one is more of the same, many interwoven stories that keep you interested in topics that you wouldn't normally think would interest you. I do have to say that now that I'm at the 80% mark, it seems like maybe it should have been wrapped up and he is stretching it to come up with new developments and sub-plots. Still, I'm hooked enough to finish it though.
For those unfamiliar is a wide, medieval, cross section of relationships (of all sorts) complicated by the struggles of commoners verses the noble elite and religious morals verses religious corruption. It's centered around Masonry of all things. Actually building, not the organization. You may want to keep a geometry set handy.
A great window into what life in the middle ages was probably much more realistically like without the romanticism of noble and shiny knights and incorruptible folk heroes and such. Enough war and violence to keep your mancard intact as well.