Postby Dragelabb » 2014-08-28, 21:30
The Norwegian language is in constant change, breeding spelling variations like these. For example, 'likne' is and older way of writing 'ligne', but it's still being used. 'Ligne' is used more often, especially among the youth. You can use whichever like, of course, but it's important to be consistent about it. It seems that you've read texts by a sloppy author.
In the case of 'slippe'/'sleppe', I've never seen 'sleppe' used in a serious text in bokmål.
EDIT: It appears you replied while I was typing. You should definitely expect material from Jo Nesbø to be of greater quality. I haven't read any of his works myself, but with all the popularity and praise he's received, I think you can consider it 'safe'.