So I was reading a novel and I don't get "你究竟是还看了些 什么东西" does it mean "what else did you see?" Or would it be more like "what else did you learn?"
Let's break up the sentence into segments: 你 究竟是 还 看了些 什么东西?
Now let's eliminate "究竟是", the remaining sentence is 你 还 看了些 什么东西? - What else you have seen?
究竟 can mean "after all", "actually" or "ever". The former two meanings fit this sentence well - After all (Actually), what else you have seen? Tang Ho's comment is correct, "是" can be dropped without affecting the sentence - 你究竟还看了些什么东西?