Anybody who seriously studies theology and/or philosophy, especially those who are Roman Catholic, must have some knowledge of this timeless classic written by St. Thomas Aquinas.
I heartily recommend this English translation of the Summa Theologica; however, if you want to do scholarly research, it would be best to get your hands on a copy of a good Latin version. A good Latin version provides access to many nuances not adequately expressed in the English translation.
Serious and orthodox Catholics, especially, will discover the Summa Theologica to be an endless font of knowledge and wisdom.
If you are a non-specialist or a person with little background in philosophical-theology and the history of ideas, it would be wise to find a companion to guide you. For this purpose, I recommend anything on the topic written by Rev. Dr. Brian Davies, OP (Oxford University), Rev. Dr. Armand Maurer, OSA (Pontifical Institute of Mediaeval Studies), Dr. Ralph McInerny (University of Notre Dame), and/or Dr. Eleonore Stump (Saint Louis University). Be careful not to read St. Thomas blindly without guides who have truly studied his thought. These scholars will provide much needed background and prerequisite knowledge.
St. Thomas Aquinas' Summa Theologica is the premier work of Catholic theology, studied at all major Catholic universities and seminaries, as well as by theologians and philosophers of religion of all denominations. Of St. Thomas' many works, this 5-volume masterpiece presents a systematic and organic treatment of several thousand important theological questions, ranging from God, the Trinity, and the nature of Christ, to the nature and psychology of the human person and the nature and mission of the Church. St. Thomas considers creation in its natural light, as well as under the operations of grace, including discussions of morality, redemption, the sacraments, and the operation of divine providence. The scope of topics is enormous, and these few hardly do justice to the contents.
The format of this work is arranged into individual articles for easy reading of St. Thomas' answers to individual questions, but there is a continuity to the way questions are raised and answered that makes the work a unified whole -- not simply a theological manual or encyclopedia, but also a profound read from cover to cover. A leading biographer of Thomas (O'Meara) has called the Summa "a cathedral of thought," which is perhaps the best description for such a short space here.
This translation has been prepared by the Fathers of the English Dominican Province. This information is not evident in the on-line description, but it should be, since it is very important that the translation of the Latin is accurate and in conformity with modern usage. (I found out by searching the ISBN at the Library of Congress website). There are many translations of the Summa available, but not all are good. The English Dominican Father's translation is one of the best editions available, and widely recommended in scholarly circles.
While the Summa Theologica is available at libraries and on the internet, it is very convenient to own your own copy of such a large work. This edition is sturdily bound with decent margins for notes, with each of the 5 volumes of a portable size and weight. (The earlier two-volume editions are large and unwieldy.)
I heartily recommend this work and this particular edition of the Summa in English