The stories may be make-believe, but also much more than make-believe (that in the sentence): It will among other teach them the morals of the agta, the myths and how they see the world. To use the correct adjective with the phrase in detail, think about fewer vs less in number vs amount - but remember in detail means specifically or completely already. What's more is an expression that's used when you want to emphasize that the next action or fact is more or as important as the one mentioned.
Understanding the Context
War doesn't bring peace; What's more, it brings. Just fyi, though, more better is pretty frequently used ironically these days by the hipsters and the whatnot to simply mean better. Also, while i think no one would responsibly advocate this.
