Rule reminder
Before-modal position: the double pronoun goes before the modal verb as a single combined unit. Example: 'Me lo puoi dare?' (Can you give it to me?). Combined forms: mi+lo=me lo, ti+lo=te lo, gli/le+lo=glielo, ci+lo=ce lo, vi+lo=ve lo.
Which correctly says 'You (pl.) must return it to me' (il libro) — pronoun before modal?