We’ve wrapped up our look at the Trinity on Monday nights, but I’ve been meaning to share the books I read in conjunction with my studying for the couple months we spent there. They are all great, but they are all different, and if you want to read one of them, the choice really depends on what you’re looking for.
Medium: The Deep Things of God: How the Trinity Changes Everything, by Fred Sanders
This was my favorite of the three, and my highest recommendation. If you just read one book on the Trinity, read this one. Sanders does an excellent job of taking you from what you already know of God in your personal salvation, spiritual walk, and bible reading, and leading you on to show you how you’ve already encountered and known God as a Trinity. (200+ pages)
Long: The Holy Trinity: In Scripture, History, Theology, and Worship, by Robert Letham 
This was the longest of the three, and the most comprehensive. If you want to cover what the Bible says about the Trinity, staritng with the Old Testament, and then look extensively at the history of the doctrine in the church (including modern theologians) and end with some application, this is for you. If you are into this sort of thing, the history is fascinating. (500+ pages)
Short: Delighting in the Trinity: An Introduction to the Christian Faith, by Mike Reeves. 
This was the shortest of the three I read. While it’s a good book, I found the other two more helpful, and I can’t recommend Reeves’ audio messages on the Trinity highly enough. But for a quick read that get’s you into the exciting idea of God’s Triune nature, this is a fine place to start. (100+ pages)
Finally, I’ll sign off with this awesome quote from Gregory Nazianzen:
Before all else, I beg you…hold fast to the confession of the Father and the Son and the Holy Spirit.
I give you to share, and to defend all your life, the one Godhead and power, found in the Three in unity, and comprising the Three separately, not unequal in substance or nature, neither increased nor diminished by superiorities or inferiorities; in every respect equal, in every respect the same.
This is the infinite conjunction of three infinite Ones, each is God when considered in Himself; as the Father, so the Son; as the Son, so the Holy Spirit.
No sooner do I conceive of the One than I am illumined by the splendour of the Three; no sooner do I distinguish them than I am carried back to the One.
When I think of any One of the Three I think of him as the Whole, and my eyes are filled, and the greater part of what I am thinking of escapes me.
I cannot grasp the greatness of that One so as to attribute a greater greatness to the Rest.
When I contemplate the Three together, I see but one torch, and cannot divide or measure out the Undivided Light.