This three-volume set is now available on Project Gutenberg in its entirety. The “New Witnesses” Roberts discusses are Joseph Smith (volume one, 1895) and the Book of Mormon (volumes two and three, both released 1909). I commented on volume one here and volume two here back when they were released.
Volume three picks up right where two left off, with further consideration of evidence for the Book of Mormon, and then goes over objections to it. Have a look!