The book's main character is Simone Simonini, a thoroughly loathsome fellow. A forger by trade, he's employed by the secret police departments of various countries to create documents and arrage sting operations that will ensare whoever happens to be the enemy at that particular moment. Many chapters are presented as entries from Simonini's diary; these alternate with sections written (and printed in a different font) by Abbé Dalla Piccola, a priest who seems to be able to recall portions of Simonini's life and career that he himself has forgotten. The two may or may not be the same person (you'll have to read the book to find out for yourself just what their relationship truly is). And an omniscient narrator occasionally breaks in to move the narrative foward a bit when the diary trails off or becomes (these chapters are printed in a third font, which is quite helpful).
Eco writes that the only fictional character in the book is Simonini himself; most of the rest of those here were real people who did pretty much what Eco has them do in the book. Only someone with as wide a literary reach as Eco would be able to pull off a book like this, with its broad overview of European history, politics, economics, religion and literary culture all wrapped up into a single character's life story. The period illustrations (most from his own collection) added throughout the text do much to enhance the text. Simonini's rapturous ravings about food were delightful to read; even though the man's a true piece of work, he can still wax rhapsodic about a good meal.
Hate is a powerful thing, and Eco has represented some of its many manifestations expertly. Not a light-hearted book, or one likely to give you good dreams, but a novel that will make you think, told by one of the smartest storytellers of our time.