Just finished reading this and...
The synopsis is somewhat misleading as the story it tells there doesn't really kick in until the latter stages of the book.
The book is told from the point of view of one of the editors 'Causabon' from his Student days in 1970's Italy, to him working with the other two editors 'Jacopo Belbo' & 'Diatollevi' at the Garramond publishing house in the late 80's and the strange cast of conspiracy theorists and occultists he meets along the way.
He is a student of Philosophy of History and is writing his final dissertation on the real story of the Knights Templar during the 1st & 2nd crusades and the eventual power struggle between them, King Phillip the Fair of France and Pope Clement III which eventually leads to their arrest & disbandment.
He's then brought in to the publishing company as an expert to check the authenticity of a manuscript based around a conspiracy involving the Knights Templar/Illuminati/Rosicrucian's etc. which leads to them becoming long term friends and collaborators
The book is over 600 pages long but feels way longer as it is incredibly dense with historical facts and exposition. The author, Eco, was a historian and a professor of Semiotics, which is the study of symbols and how they are used and interpreted. So as you can imagine, the attention to detail here is on another level, so much so that the story itself and the pacing takes a FIRM back seat to a huge amount on dialogue between characters about certain historical events, groups, people, places, dates and all that. You have to really have an interest in this kind of stuff to get through the book. Even so, I still found it a bit of a slog at some points.
I did really enjoy this over-all, but with a huge caveat that this feels more like a history textbook than an actual novel for large parts lol