The Truthful (swt) told us about the writers, here He (swt) is telling us about the Book of the wicked and the Book of the righteous.
كَلَّا (No!!): It is used for scolding and forbidding from what came before it, that is Allah (swt) saying:
أَلَا يَظُنُّ أُولَٰئِكَ أَنَّهُم مَّبْعُوثُونَ
(Do they not think that they will be resurrected. (4))
As if Allah (swt) is saying: Leave out those who deny the Judgement Day and pay attention to these facts about the Book of the wicked.
Allah (swt) called them “the wicked” because they did bad things and left out the obedience and assignment.
سِجِّينٍ (Sijjeen): It is hidden, we cannot comprehend. This word is derived from the Arabic (Sijin) which means (prison) which is the narrowest and most inferior place.