I'm looking for some supplements or novels who could give me extra info on the cult of Sigmar and his priests. Thanks!

From the novel side, I'd say Warrior Priest by Darius Hinks and Luthor Huss by Chriss Wraight.

2nd edition sourcebook Tome of Salvation is a great source on all the gods. When it comes to 3rd edition books, others can surely recommend you the right ones.