"The writer is selling his book."
Translation:Pisarz sprzedaje swoją książkę.
when using "jego" (as the second correct option) it can mean his book - someone else's or his own, but swoja would just mean his own.
To a Polish person, "jego" option, feels more like he is selling someone else's book.
In English, "his book" can have two meanings. Either he is selling his own book, or a book written by someone else.
Pisarz sprzedaje swoją książkę - The writer is selling his (own) book.
Pisarz sprzedaje jego książkę - The writer is selling his (Paul's) book.