The UN announced the book most often checked out from its library in 2015. Venture a guess as to the subject? According to the UN's Twitter account, the book is Immunity of Heads of State and State Officials for International Crimes. Vox reports that it's a doctoral thesis by the University of Lucerne's Ramona Pedretti, discussing when courts can charge foreign government officials.

According to Pedretti, heads of state can get immunity from foreign criminal jurisdiction under two circumstances: as an incumbent head of state, who can't ever be indicted by another country's courts (immunity ratione personae), and official acts on behalf of the State (immunity ratione materiae, which wears off once the official leaves office, after which they can be tried by a foreign court). When it comes to the International Criminal Court, though, things change, as the ICC has indicted sitting heads of state.

As a follow-up to the press attention its announcement received, the UN has updated its statement, says Reuters UN correspondent Michelle Nichols: Pedretti's book was only the most popular "new" book, acquired by the library in July 2015 and checked out for browsing four times and borrowed twice. (The original Tweet just said, "What was our most popular book of 2015? Find it in our library catalogue!") Still, one has to wonder which foreign delegations were whipping out their UN library cards. [Vox]