Mike,

I do not know how much of the material at Houghton Library at Harvard is duplicated at Ohio. The great majority of material is primary, some newspapers, and then typed script of Siebert's notes and some early chapters (I think this is right - it has been a while.) He also has some masters theses in the collection. I do not recall a Preston, but I did not look closely at the Ohio material because Tubman never went there. Also, if you go to Houghton, make sure you look at the volume or two which covers the southern states, because he has info in there on runaways and where they landed, as well as URR agents working in slave states but who had connections in northern states.

Kate