I've got no idea if this has been suggested yet or not, but I guess the Library should include books about Shengui Guo and Peka Glade among its rewards, such as language and tales books. And there should be a Shengui Guo and a Peka Glade history book for the Book Nook, too.

What do you guys think?

I like the idea of there being special Library reward books like the other lands. The thing is though, nobody lives in Peka so what language would it be? Maybe how to speak to your plants?

As far as anything Shegui Guo related, those could become available at Shengui Guo either on their river, or as a reading room for the bathhouse, or as part of the gift shop of the upcoming Tea House

As far as any Peka books, I think I would prefer them to be in Esther's.

I always missed that there isn't a Shengui Guo language or tale book yet seeing that it's a new region. I don't mind where the books restock whether at Five Seven Five or some newer shops. I don't know if it'll be given out by the Library seeing that all the new items are in the Quest Shop.

I can imagine the possible books on Peka Glade could be seen in Esther's next Vesnali and/or maybe Emma's expedition notes from when she discovered Peka Glade and the glade pets.