The book I am looking for is a book about a mousy girl next door and the boy next door who grew up and become someone big in technology and business. She works at a library and they are good friends. He takes her out and she convinces him to give her tips to make herself hot. She says she needs the help for a mystery man they name Mr. X. He is really Mr. X but doesn't know it. She spends some money, chops her hair, dies it cooper, gets new clothes and her mom is really upset about it because she is supporting her mom. Needless to say the makeover gets his attention and she has a show down with one of his ex-lovers who calls her the mouse. He picks her and only finds out much later that he was Mr. X.

The female character's name might have been Molly.