The most popular book among UN diplomats was about how those diplomats and their bosses can be held immune from prosecution for their crimes.
There are two types of such immunity. Functional immunity, immunity ratione materiae, says that any person who in performing an act of state commits a criminal offence is immune from prosecution. The other is personal immunity, or immunity ratione personae, which confers immunity on people holding a particular office - usually diplomats stationed abroad and their families - from the civil, criminal, and administrative jurisdiction.
So when UN diplomats receive, say, parking tickets from New York cops, they can tear them up without fear.
Oh, and they can also own slaves - and even use them as sex slaves - without worrying that they might end up in jail.
Hey - its the law.
Good to know that these public servants are so interested in learning more about international law.
(h/t Gidon Shaviv)