Of Keats and his cats

When forming a possessive to a singular noun in English, you almost without exception add 's: the cat's hats, etc. This rule applies whether or not the singular noun in question concludes with an s; hence, if we were interested in the feline companions of John Keats we would write about Keats's cats. (If we were interested in the cephaliclyptic arrangements of his feline companion, we would write about Keats's cat's hats. This could be the beginning of a peculiarly Seussian bit of literary analysis.)

Although you should never site Wikipedia as a source for anything, I will use it this time as a source for a list of more reputable sources that make this claim (including the MLA Handbook and The Elements of Style). If these aren't sufficiently authoritative sources for you, I direct you to Bob the Angry Flower's remarks on this topic.