Patience is a virtue. Perhaps less so when your life is on the line.
Things on all fronts were progressing slowly at best. David had all but given up on trying to send a letter to Celestia, and Twilight's friends had never presented an opportunity to be spoken with alone. And to top it all off, he had hit a wall in his understanding of the text held in the basement.
The circumstances required for this new plan to work were almost impossibly specific. Applejack, Pinkie Pie, or even Fluttershy would have to be at the library, alone, preferably close to night. Anything else would be far too dangerous. And with his travel only done after sundown, he couldn't exactly find a better location to encounter them alone, without scaring them half to death in the dark of night.
He was stuck. Perhaps, he needed another plan. But what was left?
Messages to Twilight: she would discover the first, and use a spell to locate him the moment he even hinted that he was in the library. Likelihood of success: minimal. Likelihood of injury: high.
Luring one of the other ponies to the library while Twilight was away: would require traveling across Ponyville and/or the gathering of many supplies. Likelihood of success: low. Likelihood of injury: medium.
Messages to Spike-
Wait.
Upon this thought, something hit him. Perhaps his first plan might not be as impossible as he thought. If he could not mix his letter with others, perhaps he could simply instruct Spike to send it, posing as Twilight. As busy as she was, such a thing was not impossible: in her rush to depart, she leaves Spike a letter to send when he is able. Although a cunning mind might be able to find such a thing unlike Twilight, he did not think the little dragon would put that much thought into it. As well, it was unlikely Spike would read the letter itself. Twilight sent many throughout the day, and he probably did not find them very interesting.
It was a flimsy plan, but as it stood, so were all his plans. There were many factors that could not be made certain. How would he make sure Spike sends the letter before Twilight returns and inspects it? How could he be sure he would not be seen, by Spike or by someone passing outside, during his time outside the basement? And-
...Did ponies have handwriting?
David did not have an answer to that one. Would Spike be able to tell that the instructions to send the letter were not written by the unicorn? It was...never brought up in the show. He hadn't a clue. The flimsiest of flimsy plans. Good enough.
The man quickly retrieved a blank piece of paper from the stacks of supplies kept in the basement. Besides the journal he had acquired when he first arrived, he hadn't a need for them until now. Quill in hand, he was finally prepared to write his letter to Celestia. Finally, his plans were moving forward.
"Dear Princess Celestia,
How did I get here I don't belong here please help me tell the ponies not to kill me and keep the pink one away from me.
A Complete Stranger,
D."
Foolproof.
David quickly rolled the parchment up as per he had seen in the show. It looked indentical. Hopefully Spike would think the same. The idea couldn't have come at a better time. Twilight was out on a picnic with her friends. With her not due back for hours, the library closed to the public while she was away, and Spike having been asleep since before she left, it was the perfect time for him to plant the letter and retreat back to the basement.
Climbing the stairs with almost cat-like grace, David slowly pushed the door open and peered through the crack to make sure the coast was clear. With no ponies (or dragons) in sight, he opened the door just enough to squeeze through. Hunched to maintain a low profile, he quickly moved to the table at the center of the room and gently placed the parchment atop it. His work done, the man maintained his stealth as best he could as he retreated, closed the door silently behind him, and breathed a quiet sigh as he returned to the basement proper.
And now, he waited. Again.
"My shirt seems to be as plainly colored as my surroundings. For that matter: I seem to be as plainly colored as my surroundings. How did I miss that until now?"