Newton’s is a part of a big, long, interesting and interlocking set of stories about subordinationists, “Arians”, and “Unitarians” in late 17th and early 18th century England. A good place to start is in chapter 29 & 30 of Wilbur’s 1925 book Our Unitarian Heritage, though his interests and biases are those of a 20th century Unitarian-Universalist.
The best account I’ve ever seen of Newton and his immediate circle is by the late great historian of theology Maurice Wiles, in Archetypal Heresy: Arianism Through the Centuries, chapter 4.
A quibble about terms – Newton thought of himself not as antitrinitarian, but rather, as anti-Athanasian, and as holding the true, primitive church doctrine of the Trinity. Thus, it can be a little misleading to describe him as “against the Trinity”, as Brandon does. It’s sort of like describing someone with a revisionary but similar (to the mainstream) historical account of the holocaust as a “holocaust denier”.