George Moses Horton's first two books of poetry are available in html format on the University of North Carolina's Documenting the American South website. Docsouth, as it's known for short, is an extraordinarily useful online resource for people working on the history of slavery-- I've found tons of great things there, not least about Horton, who's of course well covered there because of his close ties to UNC.
Horton's third book, Naked Genius (1865), is not featured there, I'm not quite sure why. It may be because of a 1982 edition published by the Chapel Hill Historical Society, which offers a facsimile of the original text from 1865, and an introduction by Richard Walser, who wrote a biography of Horton. It's now long out of print, as far as I can tell, so I'll link here a pdf.
Naked Genius is an extraordinary book, arguably Horton's best, and certainly his longest. He wrote substantial portions of it while following the Union Army as "contraband" as it passed through North Carolina. Its publication was arranged by Captain Will H.S. Banks, who apparently took an interest in Horton, and wrote the book's introduction.
It contains some of Horton's most deeply felt poems, such as "The Southern Refugee" (which casts the experience of emancipation in paradoxically melancholy terms), as well as some of his funniest, like "Jefferson in a Tight Place" (which depicts the former president of the Confederacy on the run). Horton's work here is defiantly weird, a quality he always seems to have had but ratcheted up in his old age during the heated days at the end of the Civil War.