かげには is literally "in the shadow" (kage = shadow, ni = particle denoting a location, and ha = topic marker), but here it means "behind" (If you are "behind" someone, you are "in the shadow" of that person). So your sentences mean: (1) Is it not true that, *behind* the development of extraordinary talent, there very often exists this type of creative editing? (2) *Behind* the development (of the talent) of a yet to be famous writer, there is an excellent editor. Hope this is helpful.