Read the Black Book | Main story mode - Gardener of Men TES V: Skyrim - Dragonborn Guide

After obtaining the Black Book in Nchardak, you can finally read it. Approach the pedestal with the volume and read it. The mage Neloth, who's been supporting you until now, won't help you this time, as a journey to the realm of Apocrypha is "too dangerous" for him (as he states). You're on your own.