Foucault’s Pendulum
I had a very good memory of reading this maybe 20 years ago.
This time around, I thought it started off pretty slow, but after 100 pages or so things got interesting and the last 100 pages turned into a real thriller.
The main body of the novel is the construction of the "plan" by our three heroes, who construct, for fun, a thousand year plot to find a point on the earth where they can control its magnetic forces. I haven't done my due diligence, but I'm guessing that many of the sources are real and some are invented and the plan provides a plausible account of how, since the dissolution of the Knights Templar, a central core of the knights have planned to acquire this power source and how much of European history is the result of machinations between fragments of this group.
According to the plot, he various groups - Rosicrucians, Masons, Jesuits, Baconiens, Illuminati etc - over the course of hundreds of years are all trying to find out information for where this point is and also to defame and to denounce each other.
At one point we see how the notorious Protocols of the Elders of Zion is copied wholesale from various sources that originally targeted the Jesuits or the Masons or Napoleon III.
And there are real people in the present of the story who believe in these things and come to believe in the legitimacy of the plot.
But the irony is that there is no basis to the plot - it just seems plausible - and the supposed map which will reveal the magical point does not exist.
Towards the end, Eco suggests that there is no meaning behind events that happen and that the search for such a hidden meaning (whether of God, a secret society) can only end in disappointment. The power of a secret is in its not being known and if it is uncovered then it is always disappointing unless a further hidden meeting can be discovered behind this new truth.
Eco seems to be setting up an opposition between science (provable but mundane) and magic (revelatory and exciting), but ultimately all knowledge, all systems that can be constructed are built on unstable ground.