For those who don't know, an officially licensed companion book to the MLP series was written by show writer Amy Keating Rogers, it's called "The Journal of the Two Sisters: The Official Chronicles of Princesses Celestia and Luna".

The first half of the book consists of diary entries by Celestia and Luna, depicting the events of the start of their rule over Equestria and their struggle to find their place in the new kingdom they have created. The second half consists of the letters written by the Mane Six at the end of each episode of season 4, only expanded and more illustrated.

Celestia and Luna:

The sun and moon used to be raised by teams of unicorn volunteers, who would eventually be drained of all magic for the rest of their lives by the constant strain. Celestia and Luna had no idea this was going on until Star Swirl started aging rapidly due to trying to raise the sun by himself. The alicorn sisters took on the duty of moving the sun and moon, gaining their cutie marks in the process, and used the boost in magic they absorbed from the sun and moon to cure Star Swirl and the other unicorns who had been depleted of magic.

Ever since Celestia and Luna got their cutie marks, they've been awaiting the birth of the pony whose cutie mark would match the third symbol on the Tree of Harmony — the six-pointed star.

At first Celestia refused to accept the title of "Ruler" and insisted that the more accurate description would be "Guardian".

Celestia and Luna both have the traits of the Mane Six's personalities: Celestia has Twilight's bookishness and anxiety about fulfilling what's expected of her, Rainbow Dash's love of flying fast, and Applejack's workaholic tendencies. Luna has Fluttershy's kindness and ability to understand animals, Pinkie Pie's enjoyment for playing pranks, Rarity's love of design and style, and Rainbow Dash's obsession with things being awesome.

Luna once tamed a Manticore with kindness after Celestia and Star Swirl attempt to defeat it with brute force. This surprised even her, she was never able to figure out exactly why she could do it. She eventually kept the Manticore as a pet, naming him "Melvin".

Celestia is described as a "nerd", but Luna admits that she is also the better fighter of the two, and can easily keep up with a talented spellcaster like Starswirl.

Celestia once reduced a dragon to tears through sheer force of personality.

Luna's nickname for Celestia is "Celie".

Starswirl the Bearded tried to teach Luna how to move stars(which would would later prove vital in her escape from the moon).

Celestia spent a lot of time with Starswirl in his library helping him with his spells(in reality they were actually working on a prototype of the Mirror Portal from Equestria Girls).

Celestia and Luna are not immortal instead have very very long lifespans.

Celestia and Luna did not become princesses after overthrowing Discord — instead, they reigned before Discord took over and had to retake the kingdom from him.

There were other alicorns who raised Celestia and Luna before they ruled Equestria....

Equestria:

The original Princess of the Crystal Empire is called Princess Amore, which is Cadance's middle name.

Of the six founders of Equestria, only Commander Hurricane is a stallion. Clover, Platinum, Smart Cookie, Puddinghead, and Pansy are all mares.

Princess Platinum's father was named King Bullion. She was also quite the spoiled brat, and would constantly taunt Celestia and Luna about being blank flanks. Despite this she was known to be quite brave when faced with a terrifying obstacle, as she only trembled briefly at Luna's display of magical power, he royal Canterlot voice AND her angry manticore pet before going right back to taunting her. She's certainly not a coward. She ironically ended up becoming firm friends with Luna... after being totally humiliated and ejected from the castle at high speed thanks to Luna's prank involving a trap door on the throne. She later also ended up designing the Castles tapestries...

Gregor the griffon leader's Trademark Favorite Food is eclairs.

Rhyming is apparently a racial trait of all zebras.

Starswirl the Bearded in his attempts to create time spells, instead created an age reversal spell and successfully uses it on himself. This raises the possibility he could still be....

The Mane Six:

The party for Rainbow Dash that was held in "Pinkie Pride" was in fact her "tenth birth-aversary", meaning she's been living in Ponyville for ten years.

Pinkie Pie's been trying to create and popularize a new holiday, "Lollygabbing Day"(lollygagging + gabbing), which celebrates ponies constantly talking about nothing.