Nazh is Khriss's assistant/employee, who gains a fair amount of knowledge on Roshar for her. He was referenced within the text and you get to see some of his writing in one section as well, and in some other books besides Stormlight, I think.
She's been just about everywhere, popping into Scadrial, Roshar, native to Taldain, been to Nalthis, has somehow figured out all sixteen Shardic Intents
She also writes the Ars Acanums in the end of the books, and knows a lot about what's happening at any given time, helped found Silverlight, can change her appearance via an unexplained method, will give information to anyone who asks, and knows more than even Hoid*, and she'd get along well with Jasnah.
Tl;dr: Basically she's just obliteratingly smart and has used her lifespan to gain an excess of knowledge.
*Explanation in this spoiler box here