possessives - Should an apostrophe be used with a z at the end of a word?

I know that if a word ends with an s then it should be made plural or possessive by use of an apostrophe with no extra s (e.g. "Thomas Harris' book was a riveting read." as opposed to "Gary Barlow's voice is so distracting.")

What about if the word ends with a z. For example, would it be Gomez' album or Gomez's album?

Answer

There are various views regarding proper nouns ending in s.

The Times Style and Usage Guide (2011):

apostrophes with proper names/nouns ending in s that are singular, [...] where the final 's' is soft, use the 's' apostrophe, eg, Rabelais' writings, Delors' presidency

The Guardian Style Guide (2007):

The possessive in words and names ending in S normally takes an
apostrophe followed by a second S (Jones's, James's), but be guided by
pronunciation and use the plural apostrophe where it helps:
Mephistopheles', Waters', Hedges' rather than Mephistopheles's,
Waters's, Hedges's.

In both guides pronunciation has an impact on the use of s after the apostrophe. These guides, however, only consider names ending in s. I suspect, given the guiding principles stated in the guides, that these rules could be extended to names ending in z when the z is pronounced s.

In any case, I would write Gomez's as this is not difficult to pronounce.