Twilight Sparkle is a gifted student and scholar. When she attempts to harness her creative side by writing a science-themed sensual romance novel, however, things rapidly get out of hoof. Can her friends stage an intervention before she humiliates herself (and them) at a prestigious literature recital?

No. No, they cannot.

Sweetie Belle has been living with Rarity since their parents went off to see the world, but some years have passed, and it may be time for a change.

~~~

Credit to title pic goes to dlazerous of deviantart

On a dark night filled with even darker magics, an insane cult attempts to give Nightmare Moon a body and life of her own, utterly separate from Princess Luna. But, when the spell is interrupted by Celestia, something unexpected is created.

Now, a little black alicorn filly named Nyx finds herself living under the care of Twilight Sparkle, only to be haunted by memories and emotions from her past. Is she Nightmare Moon reborn or is she simply a doppelganger with a soul and mind of her own? Can Twilight Sparkle protect Nyx from those who refuse to see past her dragon eyes and black coat?

Or, will Nyx be forced to inherit the sins that may not even be hers and become the greatest evil Equestria has ever known?

Twilight Sparkle and her fellow Elements of Harmony have faced down supernatural foes of the highest order, but sometimes the deadliest danger can come from places one would least expect. As Twilight is forced to bear a devastating secret, her efforts to keep the Elements together may well tear them all apart.
The plotline of the thrilling Pony Psychology Series continues with an exploration of deception, despair and Discord.
This series takes place between Season Two and Season Three.
Secrets and Lies shares the Pony Psychology Series TVTropes page.

“Polyamory.”

Rainbow Dash blinked at Rarity. “Come again?”

“Polyamory. What you’re trying to build with Pinkie Pie and Fluttershy is called a polyamorous relationship."

“Okay,” said Rainbow Dash, “fine. So I know what it’s called. But what I really need to know is how I’m going to make it work!”

Featured on the Pony Fiction Vault, 05/11/12!
Sequel

After an unknown accident Twilight Sparkle and Rainbow Dash awake to find themselves sharing the same body! Now they must struggle to hold themselves together as their world is turned upside down. How long can two ponies exist in a single mind? It’s a race against time as they search for answers before it’s too late. When all is said and done, you’ll never look at Twilight Dash the same way again.

She never expected to see a golden chariot flying over Ponyville. She never expected just how much her life would change. She never expected Twilight Sparkle … and Twilight Sparkle most certainly never expected Pinkie Pie.

Though the distance between Pinkie Pie and her five closest friends has grown in the half-century since they claimed their Elements, she does what she can to prevent the others from drifting away. It’s small things, mostly: tea with Rarity, a drink with Rainbow Dash, a few words with each of the others. The friendship still makes each day together magic, and she works as hard as ever to bring cheer to all of her friends. When the smiles come less frequently, that just makes each one that much more precious.

A letter from Celestia interrupts one of Pinkie's parties with disturbing news. Now friendships will be pushed to the limit as Twilight struggles to uncover which of her friends is not who, or even what, she appears to be.

(While not a romance, this does contain a shipped pair)

in Russian

Mood Wings - Observations on Pegasus Body Language.

When Twilight Sparkle finds this book, she discovers an entirely new field of research. The secrets of pegasus wings lie before her, and she insists on making her own observations on the matter.

How?

Field research, of course.