I guess you don't get out much, Anon. Here is a sampling of respectable online sources. Doing a bit of Google research yourself will turn up many, many more.
The American Heritage Dictionary:
Grammarians have sometimes objected to the so-called double genitive construction, as in
a friend of my father's; a book of mine.
But the construction has been used in English since the 14th century
and serves a useful purpose. It can help sort out ambiguous phrases
like
Bob's photograph, which could refer either to a photograph of Bob (that is, revealing Bob's image) or to one in Bob's possession.
A photograph of Bob's,
can only be a photo that Bob has in his possession, which may or may
not show Bob's image. Moreover, in some sentences the double genitive
offers the only way to express what is meant. There is no substitute
for it in a sentence such as
That's the only friend of yours that I've ever met, since sentences such as
That's your only friend that I've ever met and
That's your only friend, whom I've ever met are awkward or inaccurate.
The Columbia Guide to Standard AmE:
Although English has long and happily employed the
double genitive, as in
That lawnmower of Eleanor’s works fine, this construction, which wraps both the periphrastic genitive with
of and the inflected genitive with the apostrophe plus
s around
Eleanor
to make possession double, is now limited to our Informal and
Semiformal writing and to the lowest levels of our speech, if we use it
at all. Once again eighteenth-century argument (that one genitive is
enough, and two are improper) has at least partly won out over
exuberance, hyperbole, and redundancy. But only partly. A good many of
us do use some
double genitives and do not notice that they are double. Some language liberals argue that in Informal and Casual contexts the
double genitive
is idiomatic and not overkill, but few editors of Standard English will
be likely to let it stand in Formal writing. It’s either
friends of my sister or
my sister’s friends; even in conversation,
friends of my sister’s may grate harshly on some purists’ ears.
Random House:
The construction where a word is marked by two possessive indicators, the word of and a possessive case ('s)
or possessive pronoun, has been in the language for at least six
centuries and has been a subject of grammatical discussion for the last
two. This construction is known by various names, including the double possessive, the double genitive, the appositional of-phrase, and the post-genitive.
The main thing to remember here is that this "problem" is of almost
purely theoretical interest. No native speaker of the language has any
difficulty understanding what "I borrowed a book of John's" means.
The genitive has several different functions in English, one of which is to indicate possession. Thus, John's picture
can mean 'a picture that John owns'. However, the genitive can also
indicate other associations, so that as an objective genitive, John's picture
means 'a picture representing John; John's portrait'. This ambiguity is
one reason the double possessive is used: it allows speakers of
idiomatic English to make the distinction between "a picture of John"
(that is, a portrait of John) and "a picture of John's" (a picture
owned by John). Though your example, with "nephew," can work with or
without the 's, the "picture" example has a distinct meaning each way.
We can also note that double possessives with possessive pronouns (rather than 's
possessives) cannot be written any other way: You can say "a nephew of
John" rather than "a nephew of John's," but if you start with "a nephew
of his" you're stuck; it is completely unidiomatic to say "a nephew of
him" (though of course "his nephew" is a possibility).
Grammarians who study the double possessive have made some useful
observations. The first noun is almost always indefinite ("a picture of
John's," but not "the
picture of John's"; "friends of ours" but not "the friends of ours" or
any other specifier). The second noun is human (or otherwise animate)
and definite ("an admirer of hers" is possible, but "an admirer of the
furniture's" is unidiomatic; "of Jane's" but not "of a woman's").
The combination of indefiniteness and definiteness is not possible with
other constructions: rewriting "friends of ours" into "our friends"
makes "friends" definite, for example. In your case, "John's nephew"
means 'a specific nephew of John's', which is different from "a nephew
of John's," which means 'any nephew of John's'. As former Oxford English Dictionary
editor Robert Burchfield observes, "It is not easy to explain why such
constructions are idiomatic: one can only assert that they are."
As noted, the double possessive goes back a long way: Chaucer has "A
friend of his that called was Pandare" in the fourteenth century. The
phenomenon started to get attention with the eighteenth-century
grammarians, who generally disapproved of English constructions that
were not possible according to the rules of Latin grammar. Some of
these grammarians disapproved of the construction, while others were
ambivalent.
Among contemporary language writers, most mention the
construction but few criticize it; some restrict it to informal use,
while others call it "needed."
The Observer:
An editor asked me which of these constructions is correct: a friend of John's or a friend of John. It is idiomatic in standard English to say or write a friend of John's. I explained that we use a possessive pronoun in this construction: He is a friend of mine. I am a friend of his. Therefore, when we use a person's name in this "of" construction, we make it possessive.
This
construction is often called a double possessive, but it also is called
a double genitive. "The Writer's Digest Grammar Desk Reference" and
"The Gregg Reference Manual" point out that the use of the double
genitive can avert misunderstanding. A painting of Jennifer shows Jennifer, but a painting of Jennifer's belongs to Jennifer.
"Working With Words" cautions that the double possessive is used only to refer to people, not inanimate objects: Joan is a friend of Tina's, but not Joan is a fan of the show's. That should be Joan is a fan of the show.
Chicago Manual of Style:
Q. A friend
of John or a friend of John’s? I’ve heard that both are correct. A
friend tossed the famous ambiguity at me this way: “A student of
Einstein.” Unless it’s Einstein’s, then it might be taken to mean a
student who is working on Einstein.
A.
It is best, and, what is more, perfectly idiomatic, to use the double
genitive when “one of So-and-so’s” is what you have in mind:
a student of his (that is, one of his students)
a student of Einstein’s (that is, one of Einstein’s students)
Then
you have the liberty of writing “a student of Einstein” to mean by
contrast either someone who is working on the great theoretical
physicist as a scholarly subject or, more broadly, someone who is a
close observer of Einstein and his work.
Fowler’s notes in
its third edition that such phrases as “a student of his” are
illogical—one of the “freaks of idiom” (pp. 542–43). In any case, your
friend’s “student of Einstein” example is an excellent refutation of
those who would avoid the apostrophe s at all costs.