Princess Celestia writes letters, notes and half-thoughts intended for Twilight's viewing pleasure but often can't bring herself to send one.

(Outside the terrible, terrible accident of her well-meaning servants sending one, the fools. Thank the spirits the Twilight is so gullible, such a faithful student.)

As being a Princess, note the capital P, Celestia has the most experience to walk Twilight down the path of her destiny... But the advice that Celestia has... Simply does not translate well to the modern age.

And so Celestia writes, thinks and cringes as she tries to write a helpful textbook for her darling sucker...

Student.

She means student.