Hmm the existing skills that come later have an object category of SkillbookAirMid or SkillbookAirLate. But I don't see those inside any treasure tables.
ST_Skillbook (Levels 1-8) <- ST_SkillbookStarter
ST_Skillbook (Levels 4-15) <- ST_SkillbookEarly
ST_Skillbook (Levels 9+) <- ST_SkillbookMid <- SkillbookAirMid
ST_Skillbook (Levels 16+) <- ST_SkillbookLate <- SkillbookAirLate