Sonna is adjectival noun that express something like "such" or "that" Sonna hon wa yomima sen(in English / I don't read such books) On the other hand, "Sonna ni" is an adverb that express intence degree of things. Sonna ni takusan hon wo yondann desuka!?(Did you read books so many!?) Also, Sonna ni with a negative can express a circumstance when something was not as enough much as you thought. Kono test wa Sonna ni Muzukasiku nakatta (This test was not as much hard as I thought). Actually this is my first time teaching Japanese to someone in English.It was hard!! So,if you have an additional question,Don't hesitate to ask me again. thank you.