The Journal of the Two Sisters

My Little Pony: The Journal of the Two Sisters: The Official Chronicles of Princesses Celestia and Luna by Amy Keating Rogers is a novelization of Princess Celestia and Princess Luna's diary shown in the episode Castle Mane-ia. As a bonus, it also contains the Journal of Friendship written by the Mane Six throughout Season 4.

Coronation of the two sisters
Celestia and Luna's journal entries begin on the day of their coronation as princesses of Equestria. According to Celestia, they were suddenly approached one day by Star Swirl the Bearded and his three representatives—Private Pansy of the Pegasi, Smart Cookie of the Earth ponies, and Clover the Clever of the unicorns—to uphold the newly found harmony between the three pony tribes, believing that having rulers who represent all three tribes would be fair. Both sisters are anxious over their new positions, and note the oddness of not yet having their cutie marks at their age, but the ceremony goes on perfectly.

Building the sisters' castle
While touring Equestria in search of a place to build their castle, Celestia and Luna learn of the Everfree Forest from Star Swirl and discover the Tree of Harmony at the bottom of a ravine. Drawn to the tree's magic, the two decide to build their castle nearby. Luna draws up designs for trapdoors, hidden rooms, and secret passageways to make their home more exciting, while Celestia comes up with a library to keep her collection of books.

With the help of the three pony tribes, Celestia and Luna begin building their castle. Weeks into construction, a team of ponies fail to return from a supply run into the Everfree Forest. Luna follows the ponies' hoofprints to a cave where they are trapped by a ferocious manticore. She battles the manticore alongside Celestia and Star Swirl until the three are able to bind him with their magic. Luna is shocked to discover she can actually understand the manticore's roars, and learns from the beast, named Melvin, that he attacked the ponies because they destroyed his home to set up their new supply site. After Melvin bursts into tears and Luna explains the situation to the other ponies, everyone apologizes for the misunderstanding and Melvin's home is restored. Melvin repays the ponies by helping them finish the castle, while Luna recruits other creatures from the Everfree Forest to assist.