Kreeft’s work is more of a primer on how to read and understand the Summa. It doesn’t cover nearly enough of the Summa to be called a synopsis, in my opinion, but if you let Kreeft coach you through some of the tough bits of Aquinas’ work you may find that the actual Summa becomes much, much easier to follow and comprehend.
One thing to remember is that the Summa was not originally intended to be a spiritual treatise per se, but rather a textbook for advanced philosophy students moving on to theology. It stands alone because it is brilliant and written by a living Saint, but it was never written with the notion that a regular person off the street could read it easily. It would be like jumping from multiplication tables to calculus.
Kreefts work is excellent, but it helps to have a solid foundation in Catholic theology, and some familiarity Aristotle and Plato. Another point to remember is that Aquinas didnt sit down and write the Summa; his handwriting was illegible and he had a faithful scribe sit and listen to him work through ideas and then write them down in a Scholastic format. A good portion of it (the supplement) was written after his death by students of his. Aquinas didnt live to see the Summa completed, so his faithful students finished it to the best of their ability based on his previous lectures and notes.
The very best thing you can do to understand the Summa is ask the nearest Dominican priory for help. You may find that there are free near-college level courses available on the matter. I know there were at the Dominican Parish near me. If you do a search online for Fr. Bernhard Blankenhorn you will find a lot of great resources explaining Aquinas’ thinking, including audio and video lectures. He is the one that personally helped me read it.
Hope that helps!
Peace and God bless!
Hope that