I wrote, "nos amies écrivent des livres", and it marked it incorrect...WHY? So sexist, I suppose there are no women writers out in the literary world :-/
Until recently there were very few.
It will take more than a decade to turn around a thousand years of language evolution. Even English which is built on adaptation and diversity, only now is slowly starting to turn around. French, which is built on maintaining its purity and integrity will take a lot longer.
In french you would go for the masculine if there is one male friends amongs the group that writes books. But your answer should be accepted since there is no context
"des livres" here is only the plural of "un livre" = several books.
books are objects you can count, so: un livre, deux livres, des livres.
"some" is about something you cannot number (butter, bread...)
"Some" in English doesn't have to be about something you cannot number.
I have some books in front of me. I could count how many if I wanted, but I haven't and it isn't important, so I just told you I have "some".
To my ear as an anglophone, "Our friends write some books" doesn't sound right (though is plausible for a non-native English speaker) and the translation sounds better without it, though if the sentence were past tense you would need the word "some" ("Our friends wrote some books"). Why that is I couldn't say, but I don't think it has anything to do with countability.
In French "un livre" automatically turns to "des livres", because "des" is the plural of "un/une".
"Some" in front of a plural can be used a substitute for a plural "a/an" that does not exist in English. But generally, English plurals are used unaccompanied.
"Some/des" + plural are not about things you cannot number. Indeed you could. But you don't. So they are about "an undefined number of things".
"Some" in front of a singular, mass noun, is indeed about something you cannot number, or "an undefined portion/share/piece/quantity of something".