The Chicago Manual of Style is the standard for many publishers, and they go with the extra s, unless the noun is plural. So: Dickens's novels, but the Williamses' house. Exceptions: nouns that are plural in form, but singular in meaning (economics' forerunners), names of more than two syllables that end in an eez sound (the Ganges' source), singular words & names ending with an unpronounced s (Descartes' dreams), and for...sake expressions (their example: for Jesus' sake, but: Jesus's contemporaries).
They do acknowledge that there's controversy, adding, "Since feelings on these matters sometimes run high, users of this manual may wish to modify or add to the exceptions."
I generally try to follow Chicago unless told differently, since it's so widely used.