The Possessive Singular

What do Bridget Jones's Diary and Bridget Jones's Baby have in common, aside from sharing the same central character? Both movie titles form the possessive singular by adding 's' to 'Jones,' even though 'Jones' ends in 's.' As Strunk and White note in, The Elements of Style (p. 1), we should follow this rule "whatever the final consonant. Thus [we should] write,

Charles's friend
Burns's poem
the witch's malice"

Are Strunk and White alone? No. As the authors of Grammar Smart note (p. 122), "If the word is a proper noun that ends in -s, add an apostrophe and an -s. (This is the part that people get wrong)

Yeats's poem
Ross's riddle
Chris's crisis"

And aside from a few exceptions the Chicago Manual of Style and the Associated Press agree ("Apostrophe-S vs. Apostrophe: Forming Possessives of Words Ending in S (or an S Sound)"). Still skeptical? Consider the following "real life" examples:

Anyone who chooses to believe something contrary to evidence that an overwhelming majority of people find overwhelmingly convincing... will not be convince... And so, with this book, I do not expect to convince anyone in that boat. What I do hope is to convince genuine seekers who really want to know how we know that Jesus did exist, as virtually every scholar of antiquity, of biblical studies, of classics, and of Christian origins in this country and, in fact, in the Western world agrees. Many of these scholars have no vested interest in the matter. As it turns out, I myself do not either. I am not a Christian, and I have no interest in promoting a Christian cause or a Christian agenda. I am an agnostic with atheist leanings, and my life and views of the world would be approximately the same whether or note Jesus existed. My beliefs would vary little. The answer to question of Jesus's historical existence will not make me more or less happy, content, hopeful, likable, rich, famous, or immortal.

-- Bart Ehrman, Did Jesus Exist?, pp. 5-6

During our period at the abbey his hands were always covered with the dust of books, the gold of still-fresh illumination, or with fellowship substances he touched in Severinus’s infirmary.

-- Umberto Eco, The Name of the Rose, p. 17

But Harry was already pulling a roll of parchment from the owl's leg. He was so convinced that this letter had to be from Dumbledore, explaining everything -- the dementors, Mrs. Figg, what the Ministry was up to, how he, Dumbledore, intended to sort everything out -- that for the first time in his life he was disappointed to see Sirius's handwriting... "I can't stop the owls coming," Harry snapped, crushing Sirius's letter in his fist.

-- J. K. Rowling, Harry Potter and the Order of the Phoenix, p. 35 

Note that religion is singular in James's definition and plural in Dennett's. James is describing an experience that he takes to be universal among religions of all descriptions, while Dennett sees religions as distinct 'social systems.'

-- Marilynne Robinson, Absence of Mind, p. 8

In using it to build a science of the materially extended world, Descartes missed the significance of empirical measurement and inductive mathematical principles in physics; he went so far as to dismiss Galileo’s law of gravity because it was merely empirical. Descartes’s methodological pronouncements missed the actual procedures of the scientific revolution as badly as Bacon’s. Nevertheless, Descartes’s deductive system became for a generation or more the leading emblem of the “mechanical philosophy”; his Principles of Philosophy in 1644 was the most comprehensive statement across the range of science, incorporating everything from physics, chemistry, and physiology to celestial mechanics into a single materialist system.

Randall Collins, The Sociology of Philosophies,  p. 568

Over the course of a week Amos [Tversky] gave five different talks about his work with Danny, each aimed at a different group of academics. Each time the room was jammed--and fifteen years later, in 1987, when Biederman left Buffalo for the University of Minnesota, people were still talking about Amos's talks.

Michael Lewis, The Great Undoing, p. 205

Johann Arnason has pointed out that Jaspers's "most condensed statement" of the axial age, describing it as the moment when "man becomes conscious of Being as a whole, of himself and his limitations," and "experiences absoluteness in the depths of selfhood and in the lucidity of transcendence," is remarkably similar to Jaspers's own version of existential philosophy.

-- Robert N. Bellah, Religion in Human Evolution, p. 272 

There are plenty of more examples, but surely this should suffice.