Ok, this is what the AHD has to say about it:
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.
