Midorikawa Yuki (Midorikawa is family name and also her pen-name) is the mangaka of Natsume Yuujin-chou. She was born on 1976 and quite productive. Her other notable work was Hotarubi no Mori E. It seemed that she was interesting in shoujo manga, supernatural, and drama. If you had red or watched Natsume Yuujin-chou (including 2nd season), you could sense the supranatural and drama took the big part in the story.
Based on Baka Updates Manga, she made several mangas, from ten that had been released, these are some of them:
Akaku Saku Koe
Hana Oi Hito
Natsu ni wa Tameiki wo Tsuku
Taion no Kakera
Anyway, if you want to send a letter to her, reach her through the fan-mail:
白泉社 月刊ララ編集部 緑川ゆき様
PS: don't ask me to translate this address.. :D All that I can read is "Tokyo-to" and her kanji name :D You can print this page or copy paste the address to your word processor, then print it. Glue it on the envelope. Write at the bottom "Japan" in Latin alphabet. There! You can post it now.
Anime News Network
Midorikawa Yuki Unofficial Fan-site