There unfortunately isn't a guide to the locals laid out, just what can be pieced together from internal writings. Among shinobi, clan heads are usually assumed to either rule uncontested, or have to balance with the clan elders - it's unclear if in some clans this is a formal rule or tradition. Clan heads are usually male, as are fighters. There's some descriptions of the Uchiha clan; they're considered odd for employing female soldiers. The Uzumaki clan are considered odd for front-lining women and using seals in combat. 

The catalog of clans is sparse, mostly focused on the Land of Fire and its immediate neighbors, and is from the perspective of the Senju - it lists the Aburame (a noble clan, reclusive, use chakra-draining insects, not much else is known), the Uchiha (another noble clan, violent and spiteful, use fire and swords heavily, crow and cat summons have been observed), the Hyuuga (the third listed noble clan, can see through solid stone and behind themselves), the Inuzuka (outspoken, usually accompanied by nin-dogs), the Ino-Shika-Cho alliance (the Akimichi, Yamanaka, and Nara - who apparently focus on physical might, mind techniques, and shadow manipulation), the Chinoike clan (powerful genjutsu, suspected to have been wiped out by the Uchiha), the Hagoromo (sometimes allies of the Uchiha), the Fuuma (aggressive, known for using a four-bladed giant shuriken in battle), and Hatake (known for lightning techniques) are among the more notable clans listed. Most simply have names and last known location, with little mentioned about their techniques. 

Rian had said they had nobles, samurai, shinobi, and civilians - she would assume farmers, artisans, and merchants are the civilians, and the nobles are - their own caste? Above the castes?

She'd also said the clans were the fighters and nobility - so the nobles and the samurai would have clans, but the shinobi do so as well?

Nobles seem to be considered above the castes, shinobi below them; the noble shinobi clans are an odd exception - they're elevated above the rest of the shinobi in most authors' eyes, possibly being related to samurai clans, and are considered more honorable in their dealings.

Reading, noting.

And, she has remaining questions (about some of those power descriptions, for instance), but she's been doing this for some time, at this point - is it getting close to dinner? (she has awareness out to notice things like that.)

Yes, rather close.

She will head for the cafeteria; given lack of familiarity, a desire to gain it, and a desire to observe people further, arriving earlier is useful.

She isn't the first one there - a few people have already gathered to eat - but she's one of the earliest, and there's plenty of seats available.

She'll take a seat, not immediately next to anyone but not isolated, and look around.

She doesn't recognize anyone there except as people passed in the halls; a few of them are now identifiable as part of a specific clan - there's two Yamanaka over there, talking to a Nara, and a Hyuuga sits in one corner, glaring at her food. An Inuzuka is slipping food scraps to a dog the size of a pony. Many of the clans seem to have not been documented in what she found earlier, though.

As she has met less than ten people, she is not surprised to not see acquaintances. She notes the clan memberships, known as well as unknown. Any other traits she might fit meaning to, with her newly acquired knowledge?

That man and woman over there, dressed in ornate armor, are likely samurai who used to belong to an official clan, given their apparent wealth. There's another woman who likely has a bloodline - her skin is a light green, patterned with darker green tracery, and her eyes are gold. That person - unclear if male, female, or other from dress and appearance alone - has snake-patterned summoning tattoos on their bare arms. Another person has small storage seals on his wrists. A small group over there, with dark skin and hair in a variety of shades ranging from 'white' to 'dark brown' (and one 'blue') appear to be from Lightning Country.

Hh-hm.

Any indications of how the meal process proceeds here?

People seem to be getting food from a buffet-style wall, then bringing it back to their tables. There isn't any indication of payment needed for food, but most people are only taking a modest amount. The utensils are likely different than she's used to, but some quick observation shows how to hold and use them.

Assuming it seems there's not some division where only some people get food, she will also cross to the buffet. Scan the food for relevant information, take a comparable amount to what seems in the the standard range. 

No one tries to stop her.

The food is plain, mostly made up of the sort of thing that stores well - white rice, dried and smoked or pickled fish, various pickled vegetables and fruits, dried seaweed. There's a low variety available at the moment.

That's fine. She selects by nutritional requirements and returns to sit.

A few people glance over at her over the course of the meal, apparently recognizing a new face and at least vaguely curious.

A bit into the meal, the white-haired man from earlier walks in, Tetsuma, and walks over to her after spotting her.

"Hey. Did you find what you were looking for?"

She notes them, but won't currently come over if they do not do so first.

"Hello.

Yes, I believe I have. We arranged to meet after dinner."

"Good! Do you want company for dinner?"

"I don't mind, thank you."

"Alright, I'll be right back." He fetches food then sits across from her, and launches into light chatter about the base. He knows a lot of people, it seems, and is at least mildly curious about how she spent her day.

The base and people are of interest.

She can conversationally describe her day.

He recognizes the boys mentioned, comments that at least they seem to be getting along, and asks questions about what she read that shows a good familiarity with the contents of the library.

Getting along?

Are there any readings he would recommend or that she should take special note of?

Can he clear up any questions she has from reading, such as origins of the Sage's chakra knowledge, the Kaguya(s), and the moon?

Ah - yeah, the Tobirama and Madara take being different clans personally. Though Madara seems mostly annoyed by Tobirama in particular, he doesn't mind Tobirama's brother Hashirama.

He can recommend a few books that're good overviews, from slightly-less-biased sources. The Uzumaki have great records, and they've generally been better about staying out of all the drama on the mainland.

Okay, so - that stuff's not going to be in the library, he only knows anything about it because Rian doesn't really have a concept of 'clan secrets' and Mariko isn't apparently aware of what secrets are at all. It's not secret anymore, of course, but what had been known had been entirely a few clans' most treasured records, and there's probably gaps in their knowledge that could be filled in by things people are still hiding.

But, for what they know (and some of this is from Kaguya herself, apparently she stopped to rant at Rian at one point), Kaguya's actually from another dimension. She came here in search of the God Tree, married the leader of a major nation, and there was Drama that apparently convinced her humans are nonredeemable. She ate the fruit of the God Tree at some point, granting her chakra; her two sons were the first people born with chakra, and after she decided to take over the world the oldest was the one to notice that she was sacrificing people or something? He and his brother confronted her, she realized they had chakra and freaked out, a battle ensued. She turned the God Tree into the Ten Tails, and her sons eventually sealed her and the Ten Tails's body into the moon, the Ten Tails' chakra being sealed into the Sage himself. It's unclear if the moon existed before that or not. The Sage had two sons, Indra and Asura, who after his death fought over his legacy. Indra fathered the line that led to the Uchiha and a few other clans, Asura the line that led to the Senju, Uzumaki, and a handful of others. The Sage's brother fathered a line that led to the Hyuuga. The Kaguya clan are also descended from Kaguya somehow, they have her bone-manipulation abilities, but the records of how have been lost.