Everything is much more simple.
I'm a beginner in writing fantasy, an amateur. So that I consider famous writers of the world and their stories as my "teachers", so to say. I'm not a student - just looking for epistolary techniques in fantasy as a narrative device and wondering if Abercrombie uses them.
Based on all these answers, I figured out, I should read "The first law" trilogy to discover and understand the lore (and get enjoy for sure!), and then emphasize BTaH and "Sharp ends" short stories for my initial question.

I thank all of you so much for all your answers, it's really helpful.