I wanted to answer you yesterday but Nexon kicked me out while I was idle on the forum and couldn't log back in unless I restarted my pc >.<

Anyway,
For Rank C, it's Neil that sells you the book in Emain
For Rank B, it's Aeira, if it isn't in the shop yet, it'll be added later
For Rank A, it's Aeira again though this time, it's her "secret shop"

To access her secret shop, you need to get an Empty Treasure Box from any bears, give it to Fletta (spl?) in quest and you'll get a coupon.
With that coupon, when you talk to Aeira, you can choose to access her secret shop or not.
You can only access her secret shop once per coupon though, so you better have the money you need <_<