The APOSTROPHE of possession

This is the apostrophe that shows that someone/something owns something. You add it to a noun which otherwise does not indicate ownership: boy for example.  You do not add it to a word which already suggests ownership: hers.

There is one technique for knowing where to put it that covers ninety-nine per cent of cases and you can avoid the others by re-wording your sentence.

Turn the phrase around so that it reads: the ... of the ...  the books of the boy. Add the apostrophe and s at the end of this new phrase. Then turn the phrase back again: The boy's books
Similarly: the books of the boys. Add the apostrophe and s at the end and turn it back: the boys' books.

There is a knottier problem with the name of someone like Keats which ends in an s. Best to simply add it to the name: Keats' or leave your phrase as the poetry of Keats. Just be sure you do not put it between the t and the s or else you are calling him Keat!