Rarity is a poor reader. She is good at visuals but if you were to tell ger to write a book, it would not be very good.
In fact she actually has a library in her basement and she hardly ever reads them she just likes to look at the beautiful book covers and maybe the illustrations inside.
In fact she actually reads some comic books because she likes to see the costumes and art. (Though she would never admit to reading them.)
Or she would ask Spike to read a book for her.