PLEASE ANSWER IF YOU KNOW!! Trying to find a book...

Hi everyone!!

Well my request may be a difficult one, but maybe someone can help me. Back in high school I read a novel and I'm trying to find its name/author. It was about a young girl (described in the book as very thin and not so pretty) who falls in love secretly with a man that comes as a visitor to her house. Curiously, this man begins to develop an attraction for this girl's mother (who in the other hand, is described as really pretty).
The book is a combination of a mystery novel and a romantic novel.

That's about all I can remember.

Thanks in advance...