Numerous scholarly works suggest that the correspondence between Peter Martyr and John Calvin in 1555 is crucial for understanding Calvin's doctrine of union with Christ. Peter Martyr's letter was written to Calvin on March 8, 1555, and in it he expresses his own view and requests that Calvin explain his. Calvin's response is dated August 8, 1555. Calvin's response read in light of Martyr's letter reveals his understanding of our threefold union with Christ. Calvin distinguishes between what could be termed incarnational union, mystical union, and spiritual union (see Duncan Rankin, "Calvin's Correspondence on Our Threefold Union with Christ," in Robert L. Perry, ed. The Hope Fulfilled: Essays in Honor of O. Palmer Robertson (Phillipsburg: P&R, 2008), pp. 232-50.
The English translation of Calvin's letter to Martyr is difficult to track down. The relevant portion isn't found in the four volumes of Calvin's letters edited by Jules Bonnet. While digging around recently, I ran across an English translation of the relevant portion of Calvin's letter. It was included along with other letters in a translation of Beza's Life of John Calvin published in 1836. Because of its usefulness and historical value, I've linked the letters of Peter Martyr and John Calvin below.