How common are books on the subject of the Netherese, Netheril, Shadovar, etc.? I don't mean like actual in-game books to read, but the "invisible" books like novels, biographies, a historical study or two, things like that. I'm trying to work out how much Myv would actually know in-character with the points she has put in to the knowledge skill.
I always came to the conclusion that if you can find it in the Arabel West District Library, the information the book contains is at least decently common in the Forgotten Realms. Hydaro has read over a few Netherese books from there, so the most I could say is that while people are unsure of the specifics of how the Netherese were and it's key figures/events, their rise and fall is generally known about among those interested enough to do the research. If it is a subject that your character specializes in, I think your character would gain a bit more knowledge about it than the average individual from trips to ruins, studying relics, ect.
Most Netheril information is relatively obscure. This is reflected in the cost of the skill (as cross-class for all classes).
Just because a book is available doesn't mean that the information is "known" by most characters. For example, there are books available on the culture and history of Carthage (or even further back, Sumeria) in real life. That doesn't mean that most people are familiar with it.
Myv : I can't answer what your character would freely know. I believe the DC for just basic reading of runes in the Nether Archive is DC 15, if that gives you a guideline.
Ah, okay. So my five or so skill ranks in it is going to net me maybe the knowledge of the rise and fall.
Well, that's not so much reading the runes, as figuring out the proper way to operate the pedestals (which is why you can also UMD them, just at a higher DC).
But yes - it's pretty much a gradually increasing amount, starting with basic details, to more intricate and obscure facts.