As I have noted before, George Willis Cooke's classic "Unitarianism in America" is available free online.
I was looking for more conclusive evidence to connect Rev. Samuel Cooper to unitarianism. While the following isn't conclusive I did find it very interesting:
Alden Bradford, in his Memoir of the Life and Writings of Rev. Jonathan Mayhew, D.D., gives a list of "the clergymen who openly opposed or did not teach and advocate the Calvinistic doctrines" at the time of Mayhew's ordination, in 1747. These were: Dr. Appleton, Cambridge; Dr. Gay, Hingham; Dr. Chauncy, Boston; William Rand, Kingston; Nathaniel Eelles, Scituate; Edward Barnard, Haverhill; Samuel Cooke, West Cambridge (now Arlington); Jeremiah Fogg, Kensington, N.H.; Dr. A. Eliot, Boston; Dr. Samuel Webster, Salisbury; Lemuel Briant, Braintree; Dr. Stevens, Kittery, Me.; Dr. Tucker, Newbury; Timothy Harrington, Lancaster; Dr. Gad Hitchcock, Pembroke; Josiah Smith, Pembroke; William Smith, Weymouth; Dr. Daniel Shute, Hingham; Dr. Samuel Cooper, Boston; Dr. Mayhew, Boston; Abraham Williams, Sandwich; Anthony Wibird, Braintree (now Quincy); Dr. Cushing, Waltham; Professor Wigglesworth, Harvard College; Dr. Symmes, Andover; Dr. John Willard, Connecticut; Amos Adams, Roxbury; Dr. Barnes, Scituate; Charles Turner, Duxbury; Dr. Dana Wallingford, Conn.; Ebenezer Thayer, Hampton, N.H.; Dr. Fiske, Brookfield; Dr. Samuel West, Dartmouth (now New Bedford); Dr. Hemenway, Wells. Among those who took part in the ordination of Jonathan Mayhew, and therefore presumably of the same theological opinions, were Hancock, Lexington; Cotton, Newton; Cooke, Sudbury; Prescott, Danvers (now Salem). To these may be added, says Bradford, though of a somewhat later date: Dr. Coffin, Buxton; Drs. Howard, West, Lathrop, and Belknap, Boston; Dr. Henry Cummings, Billerica; Dr. Deane, Portland; Thomas Cary, Newburyport; Dr. Fobes, Raynham; Timothy Hilliard, Cambridge; Thomas Haven, Reading; Dr. Willard, Beverly. Dr. Ezra Ripley added the names of Hedge, of Warwick, and Foster, of Stafford. This makes fifty-two in all, but probably as many more could be added by careful search.
There's lots of other great stuff in Cooke's book. Check it out.