The most popular book of 2015 at the United Nations’ headquarters was Immunity of Heads of State and State Officials for International Crimes, according to the UN library.

What was our most popular book of 2015? Find it in our library catalogue! https://t.co/hmfeCmGKCj (UN only) pic.twitter.com/niGXUxHtGt — UN Library (@UNLibrary) December 31, 2015

The Dag Hammarskjöld library in New York announced the news on Twitter, prompting speculation about why UN staff and international delegates to the UN were borrowing a title which explores “international law relating to immunity of heads of state and other state officials in the context of crimes pursuant to international law”.

“What was our most popular book of 2015? Find it in our library catalogue!” tweeted @UNLibrary blithely, later responding to a torrent of questions: “This is an academic book concerning international laws on immunity.”

Politics and policy website Vox wrote: “The UN is full of delegates representing awful dictatorships, and the 2015 book that it says got checked out the most from the UN library was about … how to be immune from war crimes prosecution. That does not seem like a good thing.”

The book is based on Columbia University fellow Dr Ramona Pedretti’s doctoral thesis, and its publisher promises it “gives the reader a full picture of this topical issue, which is located at the heart of today’s development of international law”. Chapter headings range from “Immunity of heads of state and other state officials from the criminal jurisdiction of a foreign state” to “The immunity of heads of state and other state officials as rules of customary international law”.

“Basically, Pedretti is arguing that incumbent heads of state can’t be charged and prosecuted by a foreign court, whereas past heads of state can,” wrote Vox, quoting Pedretti’s statement that: “Immunity ratione personae prevents incumbent heads of state from being subjected to foreign criminal jurisdiction.”

The UN did not provide details on how many times the title had been checked out, but later clarified to Michelle Nichols, Reuters correspondent at the UN in New York, that it was the “most popular ‘new’ book, acquired July 2015, borrowed twice and checked out for browsing four times”, Nichols tweeted. The most popular book overall, according to Nichols, was I Am Malala, the autobiography of Malala Yousafzai.