People often forget Wesley's influence on the subject of abolition. In England, his final letter, written from his death-bed, was addressed to William Wilberforce. In it, he urged him to wrestle devils and men to remove the blight and scandal of slavery from the land. And in 1774, he penned his first work devoted exclusively to the subject. It was quickly reprinted in Philadelphia [where Benjamin Franklin was working with the first Anti-Slavery society in America] and in London. 

Wesley lost speaking engagements, financial support, and suffered the malignity of his more "bourgeois" followers. He continued to preach and reprint. It did not obscure his preaching of the Gospel, but he did not see it as separated. 

The present is the true first edition. It is exceptionally rare on the market. We have not been able to track a recent appearance. 

Wesley, John. Thoughts Upon Slavery. London. Printed by R. Hawes. 1774. 53pp. 

Side sewn at some point and removed from a sammelband. It is cropped quite closely as shown and essentially disbound, though complete.