The letter published in the Times and Seasons, was written in March 1840 while he was still living in Nashville. It was sent to Mr Howell, a pastor and friend of Fullmer's. I do not have the inclination to reproduce the letter here, since it is long, entirely doctrinal in nature and its historic value to me is only in its existence, not its content. But if you desire to read it, you can find it here. Fullmer had hoped Howell would publish the letter; in fact he claimed that Howell expresses an interest in doing so. After two years, however, he still had not, so Fullmer had it printed in an LDS publication.
Howell represented one side of the Landmark controversy (culminating in the James Robinson Graves—Robert Boyte Crawford Howell controversy in 1858–60) in which Graves supported the idea of the "exclusive validity of Baptist churches and invalidity of non-Baptist churchly acts." Howell and the majority of Southern Baptist Convention rejected this notion.
Note 1: Using Nauvoo as the place for his baptism is anachronistic. Although the saints had begun to settle there, and catch malaria, it would not be named Nauvoo until the following year.