<human>: Can you give me the gist of the text in a nutshell?

As a kid I started reading a book series, but I need your help in remembering what it is called.
I was about "magicians" in a post apocalyptic world, who searched city ruins for, what is now, modern technology.  However they lost most knowledge of the tech in this great catasptrophy.  These magicians were identified by an earring the wore with a blue ball.  I remember it started off with some street rat sneaking into a mage's house and getting caught and the mage taking him under his wing after creating some voodoo doll to threaten the boy, then dismantling it.  Any help would be appreciated.
<bot>: Magicians with blue earrings searching for lost modern technology after some great catastrophe, which caused them to lose all knowledge of modern technology.