I am writing this letter at the behest of my sister, who wishes you to come to Canterlot for Hearth’s Warming Eve.  She feels that the weeks afterward would provide a good time to teach you some of the magic which you experienced firsthoof in the Crystal Empire, as many of them are especially potent during the cold winter days and long winter nights.  I feel that Spike should be able to attend the library himself for several weeks; should he need assistance, the kind Ms. Fluttershy would undoubtedly be willing, especially as many of her animal friends are hibernating or have flown south for the winter.  Should you have time, there is a small research project which you may be able to help me with, as well.