This thesis focuses on epistolary theory in early modern England. There are a few studies of Elizabethan and Jacobean letter-writing manuals to date, though scholars typically use chronological analyses of instructional texts printed between 1568-1640. However, the methodology of this dissertation departs considerably from earlier studies. Rather than study many texts chronologically, I focus on one: Angel Day’s The English Secretary. Day’s manual, printed nine times in fifty years, was the most popular of its time. I use these editions– many of them heavily revised – to trace developments of epistolary theory. This approach necessitates a two-part methodology: bibliographical analysis and textual criticism. Before examining The English Secretary as a letter-writing text, I take up the manual, and its nine editions, using principles of bibliography to locate the revisions that Day made to his manual. Once I locate his revisions, I use textual analysis to determine their signification. In so doing, I reappraise the critical consensus about Day’s manual. It reveals that Day, typically cast as a proto-epistolary novelist or pre-Richardson Richardson, did not write as a literary author. Rather, he wrote in turns as a government servant and professional – the approved roles of a writer in Elizabethan literary culture. This newly informs the purpose of Day’s manual, as well as epistolary theory: letter-writing instruction at this time did not preview the emergence of the epistolary novel but maintained a civic, professional, and social function in early modern England.