A few days ago I found this essay, "The Logic of Trinitarian Doctrine" by Phillip Cary. It's designed for a beginner and I really enjoyed it and learned a lot from it: https://www.scribd.com/document/2385278/The-Logic-of-Trinitarian-Doctrine-by-Phillip-Cary
He has a follow-up essay here detailing more of the history of the doctrine of the Trinity: https://www.scribd.com/document/253541820/Historical-Perspectives-on-Trinitarian-Doctrine
There are also these books:
<em>The Creed: What Christians Believe and Why It Matters</em> by Luke Timothy Johnson - it's an overview of the Nicene Creed's development. I like Johnson's style alot - because he's not just writing a history, but also trying to argue that the concepts in the Creed are central to Christianity - and my church once read it for their adult Sunday School class to good benefit.
<em>The Trinitarian Controversy</em> in the Sources of Early Christian Thought series, edited by William G. Rusch - a useful and short anthology of key ancient writings in Trinitarianism.
<em>Classical Christian Doctrine</em> by Ronald E. Heine - it's not just about the Trinity but it's my favorite introductory book on the early church's theology and really solid.
ALSO: I've never read this, but it's been recommended to me: <em>Worshipping Trinity: Coming Back to the Heart of Worship</em> by Robin A. Perry