Ed Vulliamy won the prize for his book Amexica: War Along the Borderline, which documents the US-Mexican drug wars

Ed Vulliamy, the Observer writer, has won the 2013 Kapuściński award in Poland for his book Amexica: War Along the Borderline.

Amexica describes the extraordinary horror of the drug war unfolding on the US-Mexican frontier. Vulliamy travelled across the border on and off for 30 years. His reporting included a 2008 piece for the Observer magazine.

The prestigious award for literary reporting is named after the late Polish writer, Ryszard Kapuściński.

Chairwoman of the judges, Polish writer Malgorzata Szejnert, described it as an outstanding and masterful book that "conquers the imagination with language that is rich and restrained at the same time".

She added: "Amexica belongs to outstanding works of reportage that express disagreement with the madness of the world. It contains grim information but leaves the reader encouraged that there was someone brave, talented and stubborn to give testimony to the truth."

Vulliamy has been a foreign correspondent in Bosnia, the US and Iraq. He has testified at the International Criminal Tribunal against Radovan Karadžić in the Hague. His most recent book, The War is Dead, Long Live the War: Bosnia – The Reckoning, an analysis of the Bosnian war, was published in 2012.

This is the fourth year of the Kapuściński award. Previous winners were Jean Hatzfeld, Svetlana Alexievich and Liao Yiwu. Kapuściński's widow and daughter awarded the prize.