Jailed Kurdish leader Selahattin Demirtas and Turkish writer Yigit Bener on the other side of the bars use their penpal exchanges to write one of Turkey's highest-selling books. The crime novel "Duet in Purgatory", which features a retired left-wing lawyer and a bitter ageing general with a tortuous past, has been a roaring success. Co-auther Bener shares with AFP the process of the "making of" this extra-ordinary novel as well as his ideas on literature, politics and solidarity. IMAGES AND SOUNDBITES