Answer: In principle, there aren’t any. The two books were written from the level of GAR of the world of Atzilut. These two languages simply complete each other and help us in that way.
If we read books that were written only in the language of the Midrash, then it’s not clear to us as to what to attribute to them. One person might be caught in isolated details and would search for meaning in them, another would try to draw parallels, and so forth.
Therefore, in order to establish, to stabilize us with the right approach and not turn The Book of Zohar into what has been done with the Torah where everyone finds what he likes: psychology, sociology, history, law, and so on, Baal HaSulam inserted a kind of “physics” or “mechanics” of mutual spiritual activities into our learning. Thanks to this we know what he was really speaking about. His books describe a spiritual system of concepts methodically, and even though I am not there emotionally yet, I can already basically imagine the essence: the ten Sefirot, the relationship between them, their actions with regard to the upper Light found above them, how they identify with it and are filled by it.
So from the start, I can hold to the right direction without running away; after that, I can begin to clarify why the author writes like this, conveying emotions like these, using words like these of our world. Baal HaSulam communicates the upper system to me where everything is determined according to the nature of things found in the lower system, where I see the nature of the still, the vegetative, the animate nature, and humans in all their diversity and richness and all the mutual connections between them. My feelings are derived from the world in which I am found, but simultaneously Baal HaSulam describes the upper world for me: “See how it is constructed.” And so without choosing to do so, I begin to receive some kind of stimulation, begin to connect seemingly separate details, and so I advance forward.
There are many things hidden there that we do not know. When we begin to become a little more sensitive, even subconsciously, with slightly greater feeling, we receive an illumination that develops us…
From the 2nd part of the Daily Kabbalah Lesson 3/29/13, The Zohar