Do you stop reading books halfway through? I hate doing this, and can only think of three books I've stopped in the middle of:

Fyodor Dostoyevsky - The Possessed (not that I disliked it, I just started reading something else and forgot about it.)

Nick Cave - And the Ass Saw the Angel (because it's really really really really really terrible.)

Tolkien - Lord of the Rings (I didn't have the patience as an 11 year-old, and have no desire to try again now.)

I'd say Felipe Fernández-Armesto's Millennium, but I'm actually reading it, just at a glacial pace. It sits by (actually, under) my bed, and I've been getting through about twenty pages a month for the last few years. Then I'll put it down for a few months, and dip back in at a random place and start again. So I'm learning all sorts about colonization, and Mwene Mutapa, but nothing about the twentieth century or the industrial age.