Here's an excerpt from a wonderful book by Donald Fairbairn, Life in the Trinity. As the title suggests, it's an introduction to both the doctrine and practical implications of God being eternal Father, Son, and Spirit.
“One cannot speak of love and relationship unless one is speaking of distinct persons, so the distinctions between the persons are indicative of who God has always been, from all eternity. So instead of thinking in terms of One, who is somehow also three, we need to think in terms of Three, who have always been in relationship one to another and who are united in such a way that they are a single God rather than three separate gods [...]
If the link between theology and Christian life is really theosis, and if theosis is best understood as our sharing in the Son's relationship to the Father, then there must truly be an eternal relationship between Father and Son as distinct persons in order for God to share this relationship with us when he saves us. This doctrine of the Trinity is not an abstraction whose connection with Christian life is tenuous or even nonexistent.
Rather, the doctrine of the Trinity is the gateway to understanding Christian life. A God who was completely alone would have had nothing relational to offer us in salvation; he could have offered only a right status before him or something of that sort. But because God has eternally existed as a fellowship of three persons, there is fellowship within God in which we can also share.” (49-50)
Fairbairn is helpful in recalibrating ideas of theosis to respect the Creator-creature distinction. In the Spirit, we are united to share the same relationship with the Father which the incarnate and risen Christ has. He also includes many representative quotes from the early church fathers. It's worth picking up, especially if you've wondered how the Trinity fits into everyday Christian life.
Reading level: 11th grade+
Theological background: minimal; average churchgoer