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A friend gave me Foucault's Pendulum to read. It took me a very, very, very long time. And yes, it's really hard going. I had to keep backtracking and rereading bits when I lost track of what was going on.
All I can say is ... give it a go. It is worth it.
Here's what I did, and I know it sounds naff, but it works. Use a highlighter. Each time a character enters the narrative, highlight the name. Also, highlight the place where the action is happening. And in the margin (this may sound odd), write the date or time period when the scene is taking place. It will help you keep track.
If you start Foucault's Pendulum soon, and finish it by Christmas, you'll have got through it more quickly than I did. Some time, when I'm feeling particularly robust, I'm going to read it again to see what I missed the first time round. For the time being, I'm reading The Mysterious Flame Of Queen Loana, which I'm enjoying.