It was said of John Updike that he could review a book at great length without mentioning whether he liked it or not. In his diplomatic correspondence — that other literary genre — Kim Darroch does the same with Donald Trump. At no point do his leaked memos say if the president is a good or bad thing in moral or philosophic terms. Leaving such judgments to columnists and other windbags, Britain’s outgoing ambassador to Washington majors on what is useful to his masters: Donald Trump’s methods, his foibles, his prospects. The analysis is technical, not normative.