Raafi is happy to talk about clerics - they get their power from devotion to a god, or to a concept that particularly speaks to them; in his case, he's a follower of the god of travel, Fharlanghn. There are a couple dozen well-known gods and probably a few hundred obscure ones, in his world; Pelor, the god of the sun, is the most commonly worshiped. The precise role of a cleric depends on the god they're devoted to - not all gods are good, and some are directly opposed to each other - but in general they're healers, advisors, and community leaders; the type of person you take complicated problems to. Clerics also tend to be outside of society, to a greater or lesser degree; one thing all clerics have in common is that if they let mundane concerns take precedence over their devotions, they lose their powers, though different gods have different expectations for how that will be balanced. Fharlanghn's clerics, for example, are expected to always be on the move; staying in one place for a few weeks is fine, and few months is permissible as long as they don't intend to stay, but it seems that even under ideal conditions a cleric of Fharlanghn who settles for a year will lose their powers for it, while many clerics of other gods serve the same community their entire lives.
There are other kinds of divine spellcasters, too; druids are similar to clerics in that their devotion powers their magic, but they're devoted to nature and the natural world, rather than a god or a more abstract subject. (There are gods of nature, too, and clerics of it; the difference between druids and nature clerics is mostly in how much of a remove they hold themselves at.) And paladins, holy warriors, always followers of a particular god and subject to even stricter rules than clerics, and rangers, attuned to the natural world without being part of it to the degree that druids are.
Diplomacy is difficult for Raafi to give general advice about; there are several cultures and species of people, in his world, with very different ways of living and ideas of polite behavior. Humans are the most common, and unfortunately the most varied; straightforwardly hierarchical feudalism is common enough to be worth learning about, though, and Raafi gives a basic rundown of the various ranks, what they mean, and how to address them. With the technological difference, the other species they're particularly likely to want to contact are dwarves and gnomes. Dwarves tend to have extremely elaborate courtly procedures, but no expectation that outsiders will understand them: they'll be assigned a guide, if they visit the dwarves, and should be fine if they just follow directions, but shouldn't expect to come out of the situation with much more understanding than they went in with, though they can generally expect a fair and well-thought-out trade agreement to result. And gnomes don't have a nobility to do diplomacy with; each gnome town is its own entity, governed by the collective will of its citizens. Large settlements will have patron merchant-lords, but their power is purely financial, and they don't have the authority to make agreements for anyone but themselves.