Writing to youI'll write to you
At least as VonC suggests, the base structure is a bit shaped. However, I think "I write....." is somewhat unofficial in relation to "I write....". - it'?s unpleasant for me to have to deal with the more official "....to let you know...." and it turned out that the mating "I am writing to let you know...."
is much more frequent than "I am writing to let you know...." by 8,840 to 5,960. Conversely, "I am writing to let you know...." is in fact more frequent (around 17,900 to 14,000) than OP's (mixed informal/formal) "I am writing to let you know....". What is the point of initiating or writing a correspondence with me?
Significance of "Who would write to you?
It was thick and thick, made of yellowy vellum, and the inscription was in emerging gold inks. As he turned the cover over and his hands trembled, Harry saw a violet growth marker with a crest, a bear, a lion, a hawk, a dagger, and a serpent encircling a capital H.
"You' re looking for mail bombs?" and he was still looking at his note. When he gave the bill and the post card to Uncle Vernon, he would sit down and start to open the little box. and then Uncle Vernon opened the bill, gasped in revulsion and turned the card over.
When Uncle Vernon tore it out of his hands sharp, Harry was about to unfold his note, which was on the same thick vellum as the cover. "This is mine!" said Harry, trying to get it back. "Uncle Vernon grinned, shook up the note with one of his hands and looked at him.
It could mean a variety of fonts in series. If the phrase "Who writes to you? it could mean Harry already knows who they are.