Her first novel, Finding Violet Park, was published in 2007. It won the Guardian Children's Fiction Prize and was shortlisted for the Carnegie Medal. It was published in the United States as Me, the Missing and the Dead.
Her second novel, Broken Soup, was published in January 2008. It is her best selling book to date and was shortlisted for the 2008 Waterstone's Children's Book Prize and the 2008 Costa Book Children's Book Award, and longlisted for the 2008 Booktrust Teenage Prize.
A short story prequel to Valentine's book Finding Violet Park, entitled Ten Stations, was released in 2009 as part of the World Book Day short story releases.
Valentine's third novel, The Ant Colony, and a childrens story entitled Iggy & Me, were both published in 2009.
Her fourth novel, entitled The Double Life of Cassiel Roadnight, was published in 2010 and promoted through that years Hay Festival.