It can be fully packed with books. The exact weight depends on the strength of the fixings used as well as the strength of the wall that the uprights are screwed into.
One upright takes 6 fixings
6 of 6x40mm Fischer hammerfix screws (supplied) can hold 120kg (recommended load by Fischer) to 840kg (mean ultimate load by Fischer)
So if you mount 4 shelves on an upright (120kg/4) each shelf can take 30kg.