Your Mythsians remind me of a type of character myself wrote about, the "witnesses" (call me boring, but i prefer descriptive boring terms: in their own way, they give some mystical air). That said, your take on them seems way more developed than mine, that was a passing idea.
Technically, this chapter is pretty good but:
1- You left parts of the correction in this chapter (you repeat the description of the lower library twice with the same terms)
2- Sometimes, you don't notice you're using present tense while the rest of the text keeps it in past tense. Nothing that some revision can't correct.
3- I would supervise the description of the library and would enter on more detail about the appearance of the city. The first, you repeat the word "shelf" or "shelves" more times than i can count. If there's not a synonim you can use, try to go with any other way to describe the place, even if you must to use metaphors or similes. About the description of the city, i can imagine stuff (greek like city overrun by vegetation, maybe?) but you don't give us any other detail aside "marble, wood and greenery". You can't say that is greek (or egyptian, or roman for that matter) because that's something that doesn't exist in Mythsian culture (i assume) so you should predicate with the same message they use: being completely objective, even if it's a bit boring, to show that she still has some Mythsian blood inside her XD
Plot wise, you're building a story here and you don't stop to give a dissertation about everything that happens and give us that part to solve for ourselves. And that's great.
I'll keep reading. You keep writing.
Luckily, you have all your bases covered ^_^