The Summa Theologiae is a religious text written by St. Thomas Aquinas. Written then as a primer on the teachings of Catholicism.
While Thomas Aquinas has written other works, this is considered his magnum opus. In such he not only discusses catholic thought and faith but does so by referencing other religious beliefs and other great minds of his and prior ages. It is still used today by theology students and those seeking a better understanding of the catholic faith.
There’s a lot of options for this text out there right now, and honestly it depends on what you’re looking for to get the best one. If you’re looking for more of a summary understanding the Cambridge text is one to look into. It covers most of Aquinas’ Summa and at the same time tries to relate and explain its significance. It uses an older translation but has been revised more recently so you don’t need to worry about the quality of the text. In that vein, Ignatius Press has a more summary version as well. Their process is taking specific passages in the text and deep diving into their meaning, often using essays from modern scholars. This version is the best for those new to the ideas involved and wanting a concise look into the text as a whole. The University of Notre Dame Press has a version of this as well, titled The One and Many: A Contemporary Thomistic Metaphysics. The title alone tells you that this is a pretty advanced look into the Summa. It’s honestly the option I’d choose for those in a higher level theology or history class, but if you are looking more of a lighter approach this one isn’t for you. The copy by Sophia Institute Press is pretty much the middle ground option for those looking for the full original text and a summary of the meanings behind it. If you aren’t quite sure what end of the spectrum you’re on I’d recommend this one above the others as well. It’s an easy read for those new to the ideas but it doesn’t pander for those more in depth in theology or faith. The last option I’ll discuss is by Christian Classics. If you’re a professor or priest you may want to consider these. The text is broken up into five separate volumes with a striking blue and gold cover. It would look great in an office setting. It’s not all looks though. Being separated that much gives it plenty of space for the original text in its entirety. It was last translated in the late 40’s so that may be a minor issue, but for those well versed in the faith or study it shouldn’t be major. It does currently have a pretty solid price tag attached but not enough that it’s outrageous.