Recently I discussed with Jim Terman whether the possessive of "C.S. Lewis" should carry an extra "s" (Lewis' books or Lewis's books). This led to the Wikipedia article on apostrophes, which we assume is authoritative because it is, after all, the Wikipedia, ahem, source of all knowledge.
The article pointed in turn to the style guide "The King's English," by H.W. Fowler (1858-1933). The Second Edition (1908), online here, proves that snarkiness is not a recent invention of the blogosphere. Fowler says, for example, that omission of the conjunction "that" is "quite legitimate, but often unpleasant."
He also derides "Clumsy Patching" thus: "When a writer detects a fault in what he has written or thought of writing, his best course is to recast the whole sentence. The next best is to leave it alone. The worst is to patch it in such a way that the reader has his attention drawn, works out the original version, and condemns his author for carelessness aggravated by too low an estimate of his own intelligence."
[This is exactly why I re-write my stories over and over, with pen, computer and typewriter, until I either get it right or concede defeat in creative exhaustion.]
Fun reading for those avoiding actual writing.
As for Mr. Lewis, I come down hard in favor of "Lewis' " rather than "Lewis's." "Lewis' " would be parallel to the "Jesus' " construction which is preferred in Christian circles and which "Hart's Rules" (from the Oxford University Press) describes as an "accepted liturgical archaism." Sometimes it's fun to be archaic.
I was taught that we wrote "Jesus'" because there were two s's already. But I still write Walters' because consider the alternative: pronounce Walters's and it still sounds awkward. Wal-ter-zez. An extra ugly syllable. Wu's is a simple transformation from [Woo] to [Wooz], which trips as pleasantly off the tongue. So I'm being practical--no matter what the "official" rule is. If the grammarian powers that be choose to fight reason, we can join forces to change the rule!
It's always apostrophe-s unless it's one of the exceptions
The rule, as I've been taught, is that you form the possessive of a singular noun by adding apostrophe-s to it, regardless of how many esses it ends with. For example, "Brian Aldiss's novels are some of my favorite science fiction." An s-apostrophe denotes a plural possessive.
There are exceptions, but they are actual exceptions and not rules unto themselves. Most of them are Biblical names: "In Jesus' name, amen," "Moses' toeses were not roses," that sort of thing. There's St. James' Hospital in London. There are a few others. But, despite the fact that there are quite a number of these exceptions, they do not equate to a general rule that nouns ending in 's' are possessified by adding a lone apostrophe. They're irregular possessives, if you like, and they don't generalize, in the same sense that the irregular plural "mice" doesn't generalize to a rule that the plural of "house" is "hice." (And what would the singular of "rice" be?)
As a former copy editor, my main rule was always consistency within the not-so-hard-and-fast rules of grammar. All of the methods for determining whether to use 's cited above are legitimate if only because common usage justifies it. I like Strunk so I adhere to that consistently. I think Chicago Manual mentions that if 's creates 3 sibilants (-ess sounds), as with Jesus's, just the apostrophe is acceptable. Pick a way of doing things and keep to it.
Apostrophes didn't just drop out of the sky to complicate our lives-- quite the opposite. The thing to remember about an apostrophe is that it stands in for letters that have been left out, as in sec'y, cont'd, or can't.
The reason English uses apostrophe-S is because the possessive (or genitive) case used to be formed by the addition of -es to the end of the root. At one time, this was always pronounced as a separate syllable, but with time it became reduced to the sound of S or Z except after other sibilants such as S, SH, CH, etc. Because of this, the final syllable came to be written -'s, with the apostrophe indicating a missing or understood E (actually a schwa sound).
Why the same exact thing didn't happen with the plural, which was also, in many cases, formed using -es, I don't know, although obviously the distinction between balls, ball's, and balls' has turned out to be useful.
Let me further quote from George O. Curme's English Grammar (Barnes & Noble, 1947):
"English once had genitives that had no distinctive ending, so that it became common to suppress elsewhere the genitive ending. This older usage survives in a few words after a sibilant: for conscience' sake, for goodness' sake, Jesus', Xerxes', Socrates'. Some writers still suppress the -s in general after a sibilant: Cards' pride (Hugh Walpole), more commonly Cards's pride."