Ok, I am copying this here as I can't format the text in the comment:
singular:
his book - az ő könyve
his books .- az ő könyvei
plural:
their book - az ő könyvük (2 or more people possess one book)
their books - az ő könyveik - (2 or more people possess more books)
but when you have a noun (children) instead of the pronoun, you have to use the singular form
(you don't use the -k)
the children's book - a gyerekek könyve (one possession)
the children's books - a gyerekek könyvei (more than one possessions)- in this case, you only use "könyvei" and not "könyveik"
Carmelo's examples correctly would be: a barátok könyvei; a barátok tíz könyve; a két barát könyvei; a két barát tíz könyve
Hope it helps. I will need to practice explaining the grammar of my native language. ;)
I'm trying explain it in full sentences.
Ez az ő könyve. - This is her book.
Ezek az ő könyvei. - These are her books.
Ez az ő könyvük. - This is their book.
Ezek az ő könyveik. - These are their books.