Ryo Yuuki
結城稜
Background
http://fast.animenewsnetwork.com/encyclopedia/people.php?id=65663
http://www.mangaupdates.com/authors.html?id=4485
(Note first name is Ryo not Ryoh per authors website and manga covers. See image of Sheila's Diary)
From the Publisher:
Yuuki Ryoh is known for her deft ability to draw very beautiful and seductive female characters. Rumor has it that the reason the author can draw female characters so life- like and gorgeous, is because the author herself is very beautiful and she is often able to get inspiration from just looking at herself in the mirror!
(Source: http://www.amazon.com/dp/1590230019)