What is the correct possessive for nouns ending in "‑s"?

Solution 1:

Your example sentences confuse two different problems.

For nouns that are plural (such as "boys"), the possessive is formed in writing by adding an apostrophe after the plural -s. This is pronounced the same as the plural and the singular possessive:

The boys' books [boys' sounds like boys]

For singular nouns that end in -s, the possessive is formed by adding -'s, just as with other nouns. This is pronounced as if the spelling were es:

The boss's car [boss's sounds like bosses]

There is a partial exception for proper names that end in s. These names sometimes form their possessive by simply adding an apostrophe, and without changing their pronunciation:

Confucius' sayings

Jesus' teachings

However, this doesn't apply if the name ends with a letter other than s, even if it's pronounced with an s. These names form their possessive as normal:

Marx's theories

In the opposite case of a name which ends in a silent s, the possessive is usually formed by adding an apostrophe in writing, but the apostrophe causes the silent s to be pronounced:

Camus' novels [the final -s in Camus is not silent here]

Solution 2:

On singular nouns that end with an "s" or "z" sound, Wikipedia has a say. According to the article, there is no hard and fast rule on this one and different "authorities" prefer different styles.

See also St. James's park and St. James' park.