Students' books or students' book ?

[Heo]

Assuming each student has only one book,

[nt7272]

Sorry I pressed Enter and it was posted. Can't edit it.

Assuming each student has only 1 book, should I say :

"I need to collect the students' book" OR
"I need to collect the students' bookS" ?

In general, if I have : <NOUN_1 IN PLURAL>'S NOUN_2

When should NOUN_2 be in plural form and when should it be in singular form ?

Thank you very much for your help.

[Tdol]

If each student has one book, then there are plural books- if you put it in the singular, then students would be sharing a single book IMO.