Mills and Boon were the compact versions of romance novels for the ones who had grown out of The Three Investigators, Famous Five, Hardy Boys and Nancy Drew. But the romantic details in the M& B novels almost measured up to Sidney Sheldon's carnal descriptions for similar circumstances in his novels. Therefore, it was spicy and fun without a volume to carry around.
Regarding Agatha Christie, I always happened to pass her over to read Sherlock Holmes ones instead.