Twilight:Science(non fiction)
Rainbow Dash:A book based off Daring Do or some flying guide for pegasus
Pinkie Pie:Cooking guide with jokes in it.
Applejack:A motivational self help book?
Rarity:History of clothes and fabrics
Fluttershy:A novel about Fictional animals?
She is smart in being a comedian,she knows how to function as a normal pony and has some general knowledge on stuff such as how to play instruments,so she isn't stupid.