This article has details about the novel as well as its film adaptation.

Mathilukal (Malayalam: മതിലുകൾ, meaning Walls) is a Malayalam novel

Novel

A novel is a book of long narrative in literary prose. The genre has historical roots both in the fields of the medieval and early modern romance and in the tradition of the novella. The latter supplied the present generic term in the late 18th century....

 written by Vaikom Muhammad Basheer

Vaikom Muhammad Basheer

Vaikom Muhammad Basheer was a Malayalam fiction writer from the state of Kerala in India. He was a humanist, freedom fighter, novelist and short story writer. He is noted for his pathbreaking, disarmingly down-to-earth style of writing that made him equally popular among literary critics as well...

 in 1965. It is one of the most cherished and well-known love stories in Malayalam. Its hero, Basheer himself, and heroine, Narayani, never meet, yet they love each other passionately. Despite being imprisoned and separated by a huge wall that divides their prisons, the two romance each other.
The theme of the novel, focusses on the love between Basheer, a prisoner, and a female inmate of the prison, who remains unseen throughout the novel.