It is literally the first rule in the book, if the book you mean is Strunk and White's "Elements of Style": Form the possessive of singular nouns by adding an apostrophe plus "s."
What could be simpler? "The girl's dress is red." "My dog's bark is worse than his bite." "The editor's dictionary is dog-eared."
Oh, but it gets complicated. And very quickly.
The actual example of a noun forming its possessive given by Strunk and White (no, not exactly a stand-up comedy duo, though their usage guide has stood up well over time) involves somebody named "Charles."
William Strunk Jr. and E.B. White (yes, of "Charlotte's Web" fame) were trying to be provocative, I suspect - trying to put a marker down. "The girl's dress" would have been just too easy.
According to Strunk and White, Charles has a friend - who needs to be referred to, they insist, as "Charles's friend."
By choosing "Charles" as their example, Strunk and White signaled that they were parting company with another school of thought, which calls for simply "Charles' friend." With its "buzzed" "s" at the end (what a phonetician would call a "voiced sibilant"), "Charles" all by itself sounds vaguely plural, and for some people, evidently, the extra "s" is just too much.
To complicate things further, consider, for instance, Dashiell Hammett's detectives, Nick and Nora Charles, of "Thin Man" fame. In their plural form, they are the Charleses.
All these examples: Charles, Charles's friend (or Charles' friend), the Charleses, the Charleses' house - the basic noun, the possessive, the plural, and the possessive plural - are pronounced the same, "Charl-zez."
But what's really struck me lately is the way that some writers seem to follow the "Charles rule" out the window; they seem not to know when enough is enough.
They refer to the house that Jim and Sally Smith live in as "the Smiths's house."
Or the possessive of "the United States" is construed as "the United States's." Argh! "States" is already the plural of "state" (as "Charles" is not the plural of "Charle"), and plurals form their possessive with the simple addition of an apostrophe. No additional "s" is needed.
Similarly, it seems, one should be able to speak or write of "Universal Studios' latest project" or "Lehman Brothers' annual report."
And what do we do in the case of a reporter for a newspaper called "The Times"? It's a phrase that often shows up in newspaper names. It seems to suggest "the times in which we live" - our era, so to speak, or, as another newspaper name has it, "The Age." ("D'you know The Age of Melbourne?" "Oh, about 170, I'd guess; remember they had the Olympics there in 1956.") "Times" in this sense is a plural noun that doesn't have a singular any more than "scissors" do - or does.
But when a paper is called "the Times," does the "Lehman Brothers" rule apply?
Not quite. At some point there were, presumably, actual brothers named "Lehman" who can be construed as the original possessors of the firm that bears their name. But the "times" of London or New York or Los Angeles do not "possess" newspaper reporters, not even in the sense that the dog possesses its bark or its bite (see above). It's better to go with an adjectival form: "The Times reporter."
I know, by the way, I'm not making a very strong case for this other rule ("Charles' friend), but I do want to note that there is more than one way of looking at some of these issues, and that people who write "Charles' friend" do not necessarily have horns.
Such people are, however, subject to revision at many publications, including this one.
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