The first book on phrenology written by an American. Digital facsimile from U.S. National Library of Medicine at this link.

The first important American presentation of the case for polygenesis in support of slavery. Caldwell presented the first important American critique of the monogenist theories of human ancestry promoted by Samuel Stanhope Smith (No. 156.1)  and James Prichard. Caldwell "presented, with all the appearance of scientific objectivity, the case for 'polygenesis' or the separate creation of the races as distinct species . . . employing the accepted Biblical chronology of Archbishop James Ussher, [he] argued that Negroes were known to have existed 3,445 years ago, or only 743 years after Noah's ark--not enough time for a new race to come into existence through the effects of climate" (George M. Fredrickson, The Black Image in the White Mind [1987], p. 73). 

Digital facsimile from Google Books at this link.