What is the possessive of a noun ending in ‑s? Are these both right, or is the second one wrong?
the boys' books
the boss' car
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:
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:
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]
On the use of so-called 'zero genitive', marked by a simple apostrophe in spelling ('), and the 's possessive with nouns ending with an s, Quirk, Greenbaum, Leech and Svartvik specify in A Comprehensive grammar of the English Language (pp. 320 & 321) that:
In addition to its normal use with regular plurals such as boys', the 'zero genitive' is used to avoid repetitive or awkward combinations of sounds in the following cases:
(i) with Greek names of more than one syllable that end in -s, as in:
Euripides' /di:z/ plays, Xerxes' army, Socrates' wife
(ii) with many other names ending in /z/, where in speech zero is a variant of the regular /ɪz/ genitive. There is vacillation both in the pronunciation and in the spelling of these names, but most commonly the pronunciation is /ɪz/, and the spelling is an apostrophe only. (In the following examples, the minority form is given in parentheses.)
Burns' (Burns's) poem
Dickens' (Dickens's) novels
Jones' (Jones's) car
Names ending in other sibilants than /z/ have the regular /ɪz/ genitive: Ross's /ˈrɒsɪz/ theories. However, Jesus and Moses normally have the zero form of the spoken genitive, /ˈdʒi:zəs/ and /ˈməuzɪz/, and are written Jesus' and Moses' (as well as Jesus's and Moses's).
(iii) with fixed expression of the form for . . . sake, as in for goddness' sake, for conscience' sake, where the noun ends in /s/.
Boss ends in a sibilant, /s/, other than /z/, and becomes boss's in the possessive. So we have the boss's car.