shop.csv
You dont need this if you not creating new shop. Because you only adding to old one.
shopset.csv
Find correct shop column (search for older skillbook index if you cant understand). Add new book (index,X Y positions in shop). Dont forget to increase ItemCount line (count of item in that tab)
itemetc.csv
If you dont have skillbooks for new skill created already, do it. Creat new item with unused index, correct Class requirement and add skill index to it.