First, I second the suggestions of Monaldo & Sorti and C.R. Zafon.

An author who likes his "conspiracies", secret societies etc. mixed with good fun in the vein of Dumas (but contemporary) and with the techniques of the thriller, in short a kind of cultured, much better written and altogether more clever and fun Dan Brown, would be the Spaniard Arturo Pérez-Reverte. Club Dumas in particular sounds like it would be right up your alley with its mix of a mystery involving collectors of ancient books and a black magic treatise, but a lot of his books might be good choices for you. Reverte is a kind of forerunner of Zafon whom Legolas has suggested. He's less literary but he's good. He gets compared to Eco, with the Name of the Rose in mind and because he likes "scholarly" mysteries, but he's much more a real genre writer than Eco who merely experiments with genres.

Look carefully at the blurbs though, as Reverte wrote a few more historical (non mystery) novels, and he also has a "Three Musketeers" style series (Capitaine Alatriste).

Umberto Eco's most recent novel, The Prague Cemetery, deals again with conspiracies and secret societies. It's more about the Protocols of Zion than the occult in general as it was in "Foucault's".

As for classics of the genre (you'd need to enjoy 19th century pop literature, though), there's Dumas's suite of "Joseph Balsamo", "The Queen's necklace" etc. This deals with masonic conspiracies against the French monarchy, with Count Cagliostro as a main character.

I'd suggest Gérald Messadié's two volume novel about Le Comte de St-Germain (the supposed immortal man and occultist who features a lot in Eco's Foucault's Pendulum) but I have no idea if this got released in English. It's very good in the genre. The title might be something like "The Man who didn't want to die" in English.

Another 19th century classic, an Italian one, about secret societies (this one being a romantic history of the birth of the Cosa Nostra) would be Luigi Natoli's trilogy. Again, I'm not sure it's available in English. It's very old fashioned and serialesque, even more than Dumas's Monte Cristo, so not to everyone's tastes.

My favorite in the genre (mixing many genres but incl. a central big conspiracy) would be Les Gestionnaires de l'Apocalypse (The Managers of the Apocalypse), a brilliant series of 10 novels, but that one I know for a fact is only published in French.