The first volume of "Summa Contra Gentiles", "God", concerns the existence and nature of God. Although that volume requires the reader to know a certain number of technical philosophical terms, it does not otherwise require of him a great deal of contextual knowledge to appreciate.
This volume, the second volume in the work, "Creation", is more demanding. When the reader reaches the chapters concerning the intellect, the reader may well feel in reading it that he has come in on the middle of a long and complex argument. The reason that he may feel this way is because that is what he has done.
The center of the controversy is Aristotle's analysis of the intellect. His exposition on that point was not successful if we measure success by the ability of intelligent careful readers to come to a shared understanding of what he thought. Thomas's part in these controversies are the heart of this, the second volume of "Summa Contra Gentiles".
While the best thing that the reader could do to prepare himself to read this book would be to be well-read in Aristotle in general (and his "De Anima" in particular), that may not be possible for all readers. Therefore, as an aid to readers, this review will present the key terms in the controversy and what they meant (at least what they meant to Thomas)*:
Sensible - objects of sense - things that can be seen, heard, felt, tasted or smelled. Individual houses would be sensible. Contrast with "intelligible".
Intelligible - objects of reason - things that can be understood, but not sensed. The concept of "house" would be intelligible. Contrast with "sensible".
Phantasm - a sensation, whether the immediate result of the sight, hearing, touch, smell or taste of a sensible object, or a recollection of one of those sensations, or an imagined sensation. Contrast with "knowledge".
Knowledge - a correctly understood intelligible object; remembered sensations are not themselves knowledge. Contrast with "phantasm".
Memory - the repository where phantasms can be kept for later recall. Images of houses could be kept here so as to enable later recognition of them. Contrast with "possible intellect".
Possible intellect - the repository where knowledge is kept. Knowledge of what "house" means would be kept here. Contrast with "memory".
Cognitive power - sometimes used to refer to the intellect, sometimes more narrowly to the power that responds to phantasms - for example the ability to see a house, recall the image from memory, and recognize that house. Contrast with "agent intellect".
Agent intellect - the power that deals with knowledge - both in creating new knowledge from phantasms and from previously existing knowledge. Contrast with "cognitive power".
Soul - when classical philosophers debated what "the soul" was, what they were debating was what differentiated living things from non-living things. While Thomas followed Aristotle in the view that the soul was the form of the body (i.e. - what differentiated living things from non-living was not what they were made of, but how they were put together)
Nutritive soul - that most general power of the soul by which life is present in anything: its operations being reproduction and the use of nutriment. All living things have a nutritive soul.
Sensitive soul - that power of the soul through which a living thing is aware of its environment, as through touch, smell, taste, hearing and sight. The difference between animals and plants is that animals have a sensitive soul while plants do not.
Intellectual soul - that power of the soul that gives the ability to reason. According to Thomas, the intellectual soul differs from the nutritive and sensitive soul in that it is not just a form, but a substance as well, and so can exist without the body. Much of "Summa Contra Gentiles: Creation" is devoted to establishing this doctrine against competing doctrines of Plato, Alexander, Avicenna, and Averroes, among others.
Separate substances - intellectual beings without bodies, such as angels.
* In my review of "Summa Contra Gentiles: God", I included definitions for more basic Aristotelian terms than these, such as form, matter, substance, etc. Readers unfamiliar with these more basic terms might want to read that review.