So you've already read the Avatar Trilogy and the Trial of Cyric the Mad? Hm...  well, this is after Keldorn became the God of Death, so Zhentel Keep is ruins. That means Cyric's most prized artifact, a book that converts its reader to absolute worship of the god, is sealed and being carried around by one of Mystria's boys; maybe a Harper. I can't remember.

As far as the preists... well, Cyric is a god of madness, and that shows up in his followers. The deeper into his favor they fall, the less in touch with reality they usually are. As a general rule, they've got a gift for oration, and are willing to convert just about anything they can. There's a comic floating around that has one of the crazed god's clerics leading a band of Gnolls. So, as a general rule, they're crazy missionaries, but unusually aware of their role. In "Crucible," the main character (a cleric of Cyric) is clearly a fanatic, but makes some valid points; stuff like if the best in life rush off to death, that will leave only the worst, who are easy prey for his god, and so on.

If you're looking for more concrete data, if you've got a newsgroup account, you could dig around on alt.binaries.e-book.rpg. I think the title is Dieties and Demigods, but I could be mistaken.