A great book on the history/theology of the Trinity. Especially helpful are the numerous scripture quotations throughout the beginning chapters that illustrate the development of Trinitarian doctrine that is discernible in the Biblical text. Especially interesting in this regard is how Fr. O' Collins shows the differences between the Gospels and St. Paul's language when speaking of the persons of the Trinity.
The book goes all the way from the Old Testament beginnings through the New Testament, the Trinitarian controversies of the early centuries and beyond to the medievals, the Reformers (briefly), and modern philosophical and sociological currents such as the modern understanding of 'person' and even more controversial topics such as feminism's discomfort with assigning male gender to the Trinity. The important conclusion Fr. O' Collins arrives at in light of all the different ways of speaking about the Triune God is that to try to represent or explain the Trinity in any other way than how Scripture and Jesus himself explain the Trinity is to not only misunderstand it but to also divest it of both its majesty as well as its consoling presence in the life of the believer. The best way to understand the Trinity is to accept how it has been revealed.
I'm not sure this book is best for the beginner, though. Though the author does a good job at explaining the meaning behind theological concepts such as 'hypostases' and the like, it might be helpful for someone unfamiliar with those concepts to have a theological dictionary handy. Other than that, the book is very readable though at several points it can be so thought-provoking and packed with scripture quotations that you may find yourself stopping often to look up the references and soak in all that's being presented. However, at around 220 pages the book is mercifully short and does a great job at presenting a lot of material in a short space.
I dropped it to four stars only because there are sections where the author brings in some higher critical assumptions such as referring to certain of St. Paul's letters as 'deutero-pauline' when referencing those letters which I thought was a bit distracting and gratuitous. Other than that the book is fantastic.