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.