A wonderful 'friends turned lovers' romance novel is 'Roomies' by Christina Lauren. It's about a girl who needs to get married quickly for visa reasons and her friend who offers to marry her. As they go through the process, their friendship transforms into love. 'The Kiss Quotient' by Helen Hoang also has some elements of this. The main character hires a male escort, who becomes her friend first and then they fall in love. Additionally, 'Pride and Prejudice' by Jane Austen can be considered in this category. Elizabeth and Darcy start off as not really liking each other but they have a sort of underlying connection due to their social circles, and over time their relationship changes from a rather antagonistic one to a loving one which started from a form of friendship.