In all the small ways, Umberto Eco's "The Name of the Rose" is the opposite of "Foucault's Pendulum". Where FP was a fairly sprawling book, focused on heterodox and occult ideas, with a plot that spans many years and covers both hemispheres in the present day, TNotR is a more focused (though still lengthy) book, focused on orthodox Christianity, with a plot that unfolds over seven days at a Benedictine monastery in the 1300's.
Still, both books share Eco's writing style, which tends to be what I think of as "accessibly baroque." It's fairly wordy and detailed, but flows extremely well. He also has a fondness for long dialog and monologue; in TNotR, in particular, you'll often get two monks debating a finer point of theology for several pages, or someone recounting a story for most of a chapter.
One of the things I like best about TNotR is how seriously it takes religion. This is partly out of necessity: because of its setting and time, the narrator and all the characters' lives orbit around the Church and its teachings, so to do otherwise would be out of place. Still, Eco goes above and beyond, not afraid of making copious references to Biblical characters, theologians, doctrine, heresies, etc. He also works in a fair amount of history, but that history is almost always portrayed as subordinate to faith: rebellions are linked to heresies, upstart rulers to the spiritual authorities they acknowledge. I don't know if Eco is a Christian or not, but he sure knows his stuff; late in the book, a character has a dream that involves a whole menagerie of people from the Bible and the saints. It's extremely clever if you understand why each character in the dream does what he or she does. (I'm guessing that I would have gotten even more out of it if I was more familiar with the Catholic saints.)
It's difficult to pin TNotR to a particular genre. At its core, it's basically a murder mystery. The murder happens just before the start of the book, and the protagonist (a close associate of the narrator) spends most of the book puzzling out who is responsible. Still, I hesitate to call it "just" a mystery novel. While the murder drives the plot, the writing tends to be much more focused on the spiritual and political conflict set as backdrop.
The book is set around the 1320s, as Europe is starting to climb out of the Dark Ages, and I have very little familiarity with this era. From the little research I've done, it seems like Eco was mostly accurate about the larger movements afoot. The Church is the dominant power of the time: it directly holds a great deal of land and most money, and it also indirectly confers legitimacy on other European rulers, most notably the Holy Roman Empire (which, if you'll recall from your high school history class, is actually a Germanic kingdom that's far from holy). The Church is widely corrupt, addicted to the pleasures of money and power. Several reform movements within the church have sought to reclaim its earlier focus on Jesus's mission. Most notable among these are the mendicant friar movements, such as the Benedictines and, later, the Augustinians, who forsake worldly goods and devote themselves to lives of poverty and spiritual contemplation. Some of these movements are accepted by the Church, while others are denounced as heresies. The church needs to walk a fine line - the orders' focus on poverty draws attention to the church hierarchy's conspicuous wealth, and so can be seen as implicitly challenging the church; however, the new orders also serve to attract many of the faithful who would otherwise be tempted to more directly rebel against the church. The orders are on shaky ground, and depending on the current year or pope, their status may rise or fall.
Speaking of pope, the book is set shortly after the establishment of the Avignon papacy, and most of the characters we meet are deeply opposed to Pope John. They despise John for his lush lifestyle and his doctrine; one of the minor comedies of the book is how a group of (say) Franciscans will mutter "John is a heretic!", "No, a heresiarch!" And yet, he IS the pope, and the nominal head of all Christians. It's a weird dynamic, one that I don't think we really see any more today: now, we're so used to schisms that, if you disagree with a leader that fundamentally, you join or start a new church. People hop between Catholicism and Anglicanism, or vice versa; they establish offshoots that keep the old doctrine you like and omit the new innovations that you dislike. In this era, though, there's just one Church. You can try to reform it from within, but if you actually set up another faction, you'll become a heretic (no, a heresiarch), be pursued by the religious and secular arms, defeated, and burned at the stake.
All that is backdrop, but as in Foucault's Pendulum, the backdrop is the most fascinating part of the book. The setting and characters also put in strong performances. The monastery is fully realized, along with a detailed map in the book's covers; throughout the story, we get to see how the monastery functions as a community, independent unto itself but also connected with the surrounding land. The key focus of both the monks and of the book is the Aedificum, an amazing library that hosts the largest number of books outside of the Vatican, including a collection of infidel and heretical tomes. The Aedificum comes with its own rules and risks, and a surprisingly elaborate culture of obscurity has grown up around it.
Due to the large cast of characters, most of the people we meet are necessarily two-dimensional, but William, the protagonist, and Adso, his young scribe and the narrator, are very richly described. William is a man born two centuries two early; his intellectual curiosity and ontological humility grant him a perfect Renaissance temperament, which must be channeled into the acceptable profession of his day: serving as an Inquisitor and as an imperial negotiator. He also has a deep and fundamental sympathy for the positions of the Minorites, while being far too shrewd to risk associating with them in the inevitable purge. Adso is definitely subordinate to William, but makes up for his lack of intellectual rigor with a sweet disposition and earnest desire to learn.
There are way too many characters to describe here. The abbot is delightfully avaricious without being too cartoonish. Severinus is a decent and bright man, one of the few monks who can approach William on something like an equal footing. Where Severinus more or less supports William, Jorge opposes him, using his formidable skill at rhetoric and knowledge of scripture to firmly carve out their differences. Salvatore is probably the most original character in the book, speaking in a bizarre patois of his own creation, recounting some of the most awful stories within this book. And on and on - each monk has their own secrets, their own prejudices, their own agenda, and the fun in this mystery is peeling back the layers and trying to anticipate just what's going on.
I don't really have any other Eco on my to-read list, but I've been delighted by the two tomes I've already read. They're complex and intricate, but don't take themselves too seriously either. It's the kind of casual greatness that I love in writers like Neal Stephenson, and I'm glad whenever I can find examples.