"that was good. That was very good."

Couldn't have said it better myself.

Good.

Celestia stepped into the room, letting the door swing closed behind her. As always, everything else in her presence became a barely noticeable background. She was a tall mare, her skin milk white, her mane and tail flowing out behind her in an unfelt breeze. Her arms were slender, her fingers delicate. She filled an elegant grey dress to capacity, her breasts jutting out before her, her ass lightly squeezed by the small skirt into perfect shape. It ended halfway down one thigh, cut at an upwards angle. Her small feet were cradled by modest white heels, adding an inch and a half to her already impressive height. Her pink eyes sparkled as they caught the light.

I'm genuinely jealous of the way you have with words.

Lotus opened the appointments book, picking up her pen. "I do, yes", she replied with easy professionalism, relaxing from her fever-pitch of nervousness and beginning to enjoy the look of mingled excitement and terror on Celestia's face. "How does two thirty sound? It's Rosemary, isn't it?"

You're enjoying this.

"Good afternoon," Luna said, in a polite tone that nonetheless filled the room with its resonance. "I would like to make an appointment, if that is possible."

Wonder if she knows?

"Is that the manner in which my sister visits your establishment? I have witnessed how much more relaxed she seems to be after having come here, and become impressed. I would like to try your services myself."

Ohoho!

A blush coloured the royal pony's alabaster cheeks. "I may have remarked on the good care I was receiving here," she murmured, glancing to one side.

Quite.

If it had been down to Lotus, just Lotus, then she would have wanted to keep Celestia just for herself. But this wasn't about what she wanted. This was about the fantasies that the mare in front of her had nursed for so long without ever really acknowledging, or even fully understanding, what she desired. Fulfilling those fantasies, safely, and giving her space to experience the things she'd never dared do before - that was the dominant's duty.

This. This paragraph sums up what I LOVE about your writing. It's the subtle little touches that make it go from great, to fan-fucking-tastic.

Nodding, Lotus crossed the room and opened a small drawer. Removing a small section of root from beside several others, she turned back to lay it in Celestia's hands. "Chew this for a minute, then spit out the pulp. It should protect you from anything like that for a couple of days."

Fantasy world, fantasy solutions.

"Is that the manner in which my sister visits your establishment? I have witnessed how much more relaxed she seems to be after having come here, and become impressed. I would like to try your services myself."

I expect Luna would prefer the treatment with reversed roles.
But only one way to find out!

she bent toward over Lotus's desk

forward, not toward.

This is a very intriguing story, and I'd be interested to hear the story of how this all came to be, Celestia's introduction to this sort of thing.