The Holy Trinity: In Scripture, History, Theology, and Worship (2004) by Robert Letham was the third book on the Trinity and far and away the most detailed and scholarly (The first two being Delighting in the Trinity by Reeves and Experiencing the Trinity by Johnson). Letham's book provides some 550 pages exploring the Trinity for evangelicals who he argues have "underachieved" in this arena.
The book is divided into four general sections: biblical foundations, historical development, modern discussion, and critical issues. Each of these sections goes into some depth.
I as glad that he opened with the biblical foundations exploring both Old and New Testament justification for the Trinity. How these biblical foundations then gave way to developments in the early church toward the development of a trinitarian theology was quite interesting. In the second section, Letham examined how the doctrine developed and the heresies that also arose. After reaching Calvin, he jumps forward to more modern explorations, in large part because there were apparently many silent years in terms of doctrinal development.
The four chapters that made up the final section, critical issues, were the most important to me. Though the historical development was interesting and quite beneficial in terms of understanding why certain decisions were reached, this final section dealt much more with practical matters. In other words, how does trinitarian thinking influence worship, prayer, missions, and relationships. In the chapter on worship, Letham I believe rightly assesses that the modern church lacks a distinctly trinitarian worship and that we need to rethink our hymnody. Trinitarian thinking also affects how we treat others, which he explored in chapter twenty.
All in all, this is a comprehensive balanced book on the Trinity, but it is not for the faint of heart. If you are looking for a more accessible book on the Trinity, I would strongly recommend either of the others mentioned above, but if you want to go deeper, I can happily recommend Letham's text.