St. Thomas Aquinas is one of the greatest Christian thinkers (many would argue THE greatest) of all time, and is considered by the Catholic Church to be one of the Doctors of The Church. He was an extremely prolific writer, and his collected works amount to over eight million words. His magnum opus "Summa Theologica" remains one of the most influential theological works, and has been used as the de facto theological manual for centuries in the education of clergy. Unfortunately, "Summa" itself consists of one and a half million words, organized in many tightly argued logical questions. As such it is largely inaccessible to the general readership. This "Very Short Introduction" aims to introduce St. Thomas to the more general audience, and in particular to present a very concise introduction to "Summa."
The first couple of chapters of this "Very Short Introduction" deal mostly with St. Thomas's early life and the historical circumstances during which he had lived. They aim to give us a better picture of the intellectual milieu within which did St. Thomas's ideas develop. They recount several interesting anecdotes from his early life, including the one where his family tried unsuccessfully to prevent him from joining the Dominicans by bringing a prostitute to him.
The middle chapters of this book are entirely dedicated to the "Summa." Each one of the three main parts of the "Summa" gets its own chapter. The presentation inevitably manages only to skim the surface of that monumental work, but it does touch upon all the main topics that were of concern to St. Thomas. For the most part "Summa" is discussed in a very favorable way, with the full appreciation of some of the more subtle points that it elucidated. Nonetheless the author is willing to point out some of the more strange and unacceptable arguments from the point of view of the modern reader.
Since his death, St. Thomas has suffered ups and downs of popularity. Unfortunately his name had for some time been synonymous with scholasticism, but in recent decades he has witnessed a resurgence of interest as his theology has been influential on several generations of "Neothomists." If you are unwilling to tackle St. Thomas's major works in their entirety, I would strongly suggest a couple more accessible works. Peter Kreeft's A Shorter Summa: The Essential Philosophical Passages of Saint Thomas Aquinas' Summa Theologica is a particularly useful selection of some major questions from the "Summa" as well as an introduction to the form and style of St. Thomas's writing. On the other hand there is also A Summa of the Summa, St. Thomas's own abridgement. There are also several good Kindle English translations of the entire "Summa," and these a convenient way of browsing that entire work in an accessible electronic format.
Overall, this is a great time to learn more about St. Thomas Aquinas, and this Very Short Introduction is a wonderful first step in that direction.