[Complete! With TVTropes apparently!]
Also, apparently, an animated trailer.

In a world of brass and steam, Twilight Sparkle had thought she had made a life-changing discovery with the invention of the telescope. For better or worse, she was correct.

Now her discovery has not only changed her life, but the lives of those she seeks out in her desperate attempt to contact the only other creature as lonely as Twilight herself.

It all would have been much simpler, but it had to be the one Twilight could only call The Mare on the Moon.

Decidedly not within walking distance, then.

[Featured on EQD before story was meant to be submitted. 4/03/2015]

[Editing team: Southpaw, Wolfvenom, Maskedferret, SwanSong and BluePaladin42, without whom this story would be a fraction of its current state.]
[Special thanks to Novel-Idea for the updated cover]

Space Princess Luna returns to New Canterlot to charm her love and root out a dangerous spy known only as The Broker. A tale of space, spears, and thrilling intrigue!

This is a planetary romance and a love-letter to the works of Jack Vance, Robert E Howard, and Ray Bradbury. Expect high camp, purple prose, and space duels.

Twilight becomes alicorn of the stars. This is sort of a problem, because Luna kind of already was alicorn of the stars. Oops!

TV Tropes

This story is a sequel to Off The Edge Of The Map

When Twilight is sent with Luna on a diplomatic mission to Draconia, they quickly find themselves somewhere that doesn't appear on any map, and Luna is lost in more than one way...

Thanks to RBDash47 of Pony Fiction Archive for formatting it for submission.
Title image by Valcron

Tremendous thanks to Illya Leonov and Scribbler for the Audiobook version.
Now with a Spanssh translation by SPANIARD KIWI: https://www.deviantart.com/spaniard-kiwi/art/Apoteosis-2-3-857110089

Right now, I'm in an unfamiliar bed, but even so it feels quite comfortable. I'm relaxed and safe, but I think that's because of the pony laying next to me. It's been over twenty years since I met her, and along with her there are a lot of ponies in the room with me. They're all really nervous and scared. But I'm okay, because she is here.
My name is Twilight Sparkle. I am the Archmage of Equestria, the youngest ever at forty five years old, and right now, I'm laying on my deathbed.
But that's okay.
As long as Luna is here, I'll be okay.

Princess Luna knows a thing or two about nightmares and dream magic. Her duties at Night Court have become tedious. With the help of some forbidden power she reaches out to a certain Element of Magic for the companionship and knowledge of modern Equestria she desires. But while Luna is distracted and Celestia sleeps, an old evil returns with plans both dire and terrible and a new evil plots under their noses.
(Story takes places roughly between season 2 and season 3, so post-changlings, pre-Sombra)

Cover image by JJ at lovetomorrowlove.tumblr

AU. Based on the first season. Twilight and Celestia begin to drift apart due to a misunderstanding. Will Luna speak up and repair the breach, or will she remain silent to keep her new friend, Twilight, all to herself? What other challenges will face everypony involved because of this new relationship?

It's a simple little love story. Unfortunately, love's not simple.

Small stories and drabbles that focus on the faithful student and the estranged sister.

Luna and Twilight are sunk into a fractured, feudal world, where the knowledge of magic is lost. Penniless and depleted, they're forced into playing by other ponies' rules, all the while trying to discover even where home is.

Princess Luna finds more than a friend in her sister's student, and Twilight Sparkle gets a chance to study the moon and stars far closer than she ever dreamed possible.

An ongoing series of slice-of-life snapshots. Expect a castle's worth of clop-free d'aaawww.

These stories are in continuity with The Rustic & The Romantic and If the Flight Suit Fits.

Now featuring cover art by the inimitable Cerraka (formerly xaztein / crenair)!

Almost all chapters originally written as separate entries for flash-fic blog Thirty Minute Ponies (TMP). I highly recommend a visit to TMP if you're looking for some of the best 1000-word gems in pony fiction. Many thanks to TMP for inspiring me to write!

Plan C - In which Cadance learns that Celestia is the chessmaster, and that her backup plans are the most fun of all.

Game, Set, Match - Twilight tries her hoof at arena combat. Victory or defeat, she's sure to end up a winner with this opponent.

Countdown - Luna swore she'd never do this again, but there's something about that little librarian from Ponyville that tests her resolve.

Twilight Hustle - Rarity's heard some steamy rumors about Twilight and gathers the girls to dish juicy gossip.

All the World’s a Stage - Luna had a wonderful evening with Twilight and sang her heart out about it. Oh, and she might have left the door open.

Every Morning After - Twilight and Luna shared a wonderful, magical night; unfortunately, immortal Princesses sometimes forget about thousand-year-old laws which could ruin the mood ever so slightly.

Names - Luna's had many more names and titles than "Mare in the Moon", and Twilight discovers which of them meant the most.

Great Minds Think Alike - Cadance and Celestia's plan for Twilight and Luna comes to a head, slightly off schedule.

Third Time's the Charm - Twilight's Nightmare Night costumes aren't cutting it. Now Luna's in town, and that may change her opinion on what makes a proper costume.

Monster - Ponyville's been through the ringer lately, but whose fault is it really?

Impossible - Luna's having a bad night, and Twilight came all the way to Canterlot to do something about it.

Early Observances - If you're going to interrupt Celestia's sleep, you'd better have a very good reason.

THAT Letter - Luna and Twilight have to tell Celestia how serious their relationship has become, and they decide to send a letter. Surely they couldn't disagree about how to make such a simple announcement - could they?

Your Turn - Twilight and her friends reveal some personal secrets, and Twilight's is life-changing.

Trembling Joyful Whispers- The Big Day is here for Twilight and Luna! Hear the congregation's comments as events unfold.

The Power Vested in Me- The Big Day is here, but Luna and Twilight are so very, very far from ready. Will their fears get the better of them?

Awkward - Twilight and Shining Armor have dinner with their parents along with Luna and Cadance. They'll probably wish they'd ordered takeout instead.

Absence - Luna deals with a thousand years of angst all at once; maybe she just needs a gentle helping hoof to make sense of it all.

You Were There - A quiet night between Luna and Twilight is almost derailed by a lesson in ancient mythology.

Between Sun and Moon - Luna reads a bedtime story with which she is all too familiar.

Postcards from the Edge - Twilight leaves to study abroad for three months; in her absence, Luna has a stack of mail to open.

Insert Princess T into Slot S - Twilight want to be Princess of Something. Careful what you wish for!

Ups and Downs - Newly-crowned Princess Twilight needs flying lessons. Careful whom you ask!

Amethyst Throne - Princess Twilight is pretty sure she's seen these petitioners somewhere before.