I am a ice wizard level 50 and still cant get the Frost Giant Spell. I went to Professor Greyrose and she doesnt say anything to me. I look at my quests and dont have an outstanding quest from her.
perhaps you already accepted the quest but you havent realized it. lvl 48 quests have multiple parts so you may not have recognized the "new spell" icon as a reward for the quest.
Did you do your level 38 spell quest?, Did you do your level 28 spell quest?,Did you do your level 18 quest?,etc. If you haven't done either of them you must do them in order to get the next quest.