The thesis of "Joseph Priestley and English Unitarianism in America" is found on p. 10: "this book seeks to restore Joseph Priestley and English Unitarianis to their proper and influential place in the history of the American denomination. . .", and this reviewer thinks that in this JD Bowers largely succeeds.
On p. 18 of the book, Bowers quotes Priestley as calling the doctrine of the Trinity "absurd;" but absurdity is in the mind of the beholder. Just because Priestley could not fathom the Trinity does not ipso facto make it absurd. And one would take mystery out of religion, mystery understood as a religious truth about which accurate statements can be made, but that the non-omnicient human mind cannot totally comprehend, thereby signs onto the dogma of rationalism, which they usually don't recognize as a dogmatic belief, because they take take it for granted as common sense.
On p. 46 we learn that in the eyes of Priestley, American Unitarianism is Arminian with a touch of Arianism, which states that Christ is divine, but not equal in status to God the Father. Priestley leaned more toward Socinianism, which considers Christ as a mere man, and he thought this variation more rational and in coherence with the Scriptures.
Bowers shows how Unitarianism in America, particularly in New England, grew as a reaction against the emotionalism of the Great Awakening. It was also used as a theological weapon against the Catholics of Quebec, and the quasi-Catholics which many American Protestants considered Anglicans to be. Ironically, one could state that Priestley played the role of pope to some Unitarians. Priestley supported the French Revolution, a highly significant point which Bowers dispatches with one line on p. 88.
On p. 144, Bowers notes that John Sherman, who succeeded Priestley at his PA church, declared that this church would have no doctrinal tests for admission, but isn't the denial of the Trinity a major doctrinal test?
All in all, this book is worth reading if you are interested in the very important role of Unitarianism in the formation of the USA. Some think of Boston as a Catholic city, but it was only such for maybe one hundred years. Before that, it was Unitarian, and is becoming informally Unitarian again.