Computer Science? About two guys who own a rare bookstore in the 90's, someone comes in asking for a "how-to" manual to build a computer program/computer, which doesn't exist so they make them up to sell, then the man declares they actually work

[Willow]

Unsure if the end result is that the man built a computer or maybe a program?

I believe it's basis was around how computer logic was formed.

Possibly with "Pendulum" in the name?

[Capn]

I'm wondering if maybe you are perhaps thinking of the Umberto Eco book, Foucault's Pendulum.

NY Times says: ''Foucault's Pendulum'' is an encyclopedic detective story about a search for the center of an ancient, still-living conspiracy of men who seek not merely power over the earth but the power of the earth itself, and who in the end draw their pursuers into a circle where discovery of the truth is lethal ...

You can check it out for free here:
OpenLibrary: https://openlibrary.org/works/OL14852...

Searching for "computer":

There were two rooms in all, books piled in every comer, shelves sagging under their weight. The table with the computer, printer, and boxes of disks. A few pictures in the space not occupied by shelves. Directly opposite the table, a seventeenth-century print carefully framed, an allegory I hadn't noticed last month, when I came up to have a beer before going off on my vacation.
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That was the only file that had been printed out. I would have to go through the disks on the computer. They were arranged by number, and I thought I might as well start with the first. But Belbo had mentioned a password. He had always been possessive with Abulafia's secrets.
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But maybe not: a term associated with the Tradition might also occur to Them. Then I thought: What if They had already broken into the apartment and made copies of the disks, and were now, at this very moment, trying all the combinations of letters in some remote place? Using the supreme computer, in a castle in the Carpathians.
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Nonsense, I told myself. They weren't computer people. They would use the notarikon, the gematria, the temurah, treating the disks like the Torah, and therefore would require as much time as had passed since the writing of the Sefer Yesirah. No, if They existed. They would proceed cabalistically, and if Belbo believed that They existed, he would follow the same path.

It's sort of an all encompassing conspiracy-theory jaunt into madness.... :) Abulafia is the name given to the computer, with its files of randomness, that sort of starts making sense and maybe... well, I don't want to spoil it.

The computer's importance, I believe, is mostly limited to the first 30 pages or so - it contains the files with the data that drives the story going forward. The story is set in Italy (Milano, etc.). There is a part about unscrupulous publishers and luxury editions, and privately published work on the Rosicrucians, Templars, etc.