This story is a sequel to Bring Them Back

Twilight tried to put everything back the way it was; her old friends rose from the grave, her old dragon now sits on her back again, a baby once more. But some things always change. Her actions in the Everfree have consequences, and as she emerges from the forest, she will discover a new Equestria, held captive by the perilous Wilder. Airships patrol the skies and artillery rings out in the night. This is not how it should be.

Twilight's journey to set things right will take her through the wilderness, over stormy mountains, and
into the treacherous Black Zones, but she will not go alone. With Spike by her side and her friends in her heart, Twilight will do what it takes to save Equestria.

A simple accident with magic results in a pony claiming to be an accountant appearing on Twilight Sparkle's door-step, and that is just the beginning of an epic tale of revenge, laughter, chaos, family, petty squabbles and on a few occasions, true love, spread across time, space, and entire universes.
Well, sort of.

First time writing. Constructive criticism welcome.

Now has a TVTropes page

Twilight receives a letter out of the blue that reveals a secret she's been keeping from her friends, a letter that conveys the condolences of a family friend. What hidden meaning does it carry, and how will Twilight deal with the responsibility that comes with it?

After Twilight's grim duty is completed, the mane 6 travel out of the country on a diplomatic mission to the Griffon territories and find trouble on the way.

(If there are grammatical or other errors, please tell me in the comments and I'll fix them)

Five thousand years have come and gone since Twilight took the crown and became an alicorn. The centuries have not been kind to her; they took away her friends, one by one, till only she is left, an immortal ruling an eternal kingdom.

Now, even her memories are fading, her recollections slipping as she struggles to recall the only happy days in her life, the Pinkie Pie parties that ended too soon, the sound of a certain pegasus's voice.

Twilight Sparkle wants them back. And there is one spirit she thinks might be able to help her.

A year after the Battle of the Bands, high school graduation is nearing and the girls are ready to face the world, though Sunset Shimmer worries that they might drift apart. But when Twilight's human self arrives and ends up getting infected with dark magic, they must call upon Princess Twilight once more to engage in a battle that will decide the fate of their worlds.

Tagline: A Duel of the Duplicates

BTW this is how I thought the third Equestria Girls movie was going to go.

Covert Art done by Eveyann.

This story is a sequel to The Monster in the Twilight

Now that Luna has returned to the throne, and the aftermath of her brief appearance as Nightmare Moon is being cleaned up, the story is over. Right?

Not quite.

Twilight Sparkle had nearly been destroyed in the chaos of Nightmare Moon’s return, but now with the assistance of her five friends, she is facing a fascinating new world filled with wonderful things. No longer the scarred monster who nearly destroyed Canterlot and raged through the Everfree Forest for over a decade, now she has been healed by the Elements of Harmony and is living at the edge of town with her adoptive mother Zecora. Not all the fears of her past can be left behind that easily though, and now she must face the consequences of her actions as she faces the most daunting challenge of her life.

Ponyville.

Letters is a sequel to the alternate universe story The Monster in the Twilight. In this story, Twilight Sparkle exchanges letters with Princess Luna, two ponies with a common problem of fitting in and making friends after the destruction of Nightmare Moon.

Normally I would try to talk you into reading the sequel even if you haven’t read the original story, but in this case, you’ll probably be more confused than entertained. But go ahead. I don’t mind if you don’t read the first story. You’ll never find out what an Urlock is, but that’s fine. It’s your funeral.

Editing Assistance by Alicorn Priest, Peter and Tek

Credit for the incredible picture All Night Study Group goes to Bibliodragon at Deviantart

Physical books are available on Lulu.com (up to Chapter 82)
Letters From a Little Princess Monster (Paperback) 726 pages
Letters From a Little Princess Monster (Massive Hardback) 780 pages