The novel, Filer's first, charts the story of two brothers split by a boyhood accident, taking the reader into the surviving sibling's mind as he struggles to deal with mental illness and the fallout from his brother's death.
Filer—who has only just been married—told the BBC the story was tinged with sadness but he believed the main character was one that readers would like to spend time with.
Filer's book is published in the United States as "Where The Moon Isn't."
The Costa Book Award is worth 30,000 pounds (roughly $50,000). It's open to writers based in Britain and Ireland.