It's not often that a dusty collection of academic texts make the headlines, but a recent tweet from the UN's Dag Hammarskjöld Library has certainly bucked that trend.

Among the documents and publications held at the UN's main library in New York is a book called "Immunity of Heads of State and State Officials for International Crimes".

Published in 2014 and written by Ramona Pedretti, a Swiss legal expert, the book turned out to be the most popular among UN diplomats last year: