Where does the book sky clans and the stranger go in the warrior series?

The SkyClan story goes simultaneously with the regular series. After series 1 in the intermission into series 2 is when you should read Firestar's Quest. Then right after that you could continue with SkyClan's Destiny then SkyClan and the Stranger or you could read Midnight (series 2) Depending on what order you wish to read it in.