I do have this book but please let me know if you come across anything that may help with James and Mary Silver/s Letts in particular. I am sure there are plenty of sources out there that I may not have seen yet. I have found data on some other NJ families in Warren Co. in "The History of Warren County Ohio Part V. Biographical Sketches Union Township" (Chicago, IL: W. H. Beers Co, 1882; reprint, Mt. Vernon, IN: Windmill Publications, 1992) Page 1061. Familiar surnames are Perrine, Budd, Randolph, Laurence/Lawrence, Rogers, Lippincott, Engle, Silver/Silvers, Stokes, Barkalow/Buckelew, Wickoff/Wycoff, Tice, Cox, Borden, Crane, Emley, French, Hunt, Burrows, Haines, Lamb,
"ISAAC CAMPBELL, farmer; P. O. Oregon; was born in Salem Co., N. J., June 1, 1824, and is a son of James and Temperance (Bradford) Campbell, also natives of Salem Co., N.J." (note this is were Hannah "Silver" Letts's family was from) I know your Campbell's were from NY but Andrew found his way in to Hannah "Silver" Letts's family. (They owned a bit of land and in many parts of NJ.)
All these names are associated with the Monmouth, Ocean and Burlington Letts's either by marriage or location in NJ. So I feel this James Letts c. 1811 belongs with one of our Letts lines. I presently have James as a potential son of Campbell Letts son of James and Hannah Campbell with this James being son of Peter and Hannah "Silver" Letts. I am also aware of the distance between Ashtabula Co. and Warren Co. Ohio. I could be way off but it feels like a comfortable fit and deserves further research.
Thank you and keep in touch.