I would distinguish between challenging, because book is so overwhellming because of high level of proficiency required to read with understanding (eg. Universi

I would distinguish between challenging, because book is so overwhellming because of high level of proficiency required to read with understanding (eg. University paperbooks) so here I could say that a lot of was hard thus challenging. Other kind are challening because it change way you think about world and/or things and it's hard to apply knowledge you get, even you know it's right "view" like for me some of buddisth papers and Enchiridion by Epictetus. It's pleasure to read, but when you want to live in stoic way - yeah, then it's challenign.