The book is subtitled "An old time epistolary novel by seven fictitious drolls & dreamers each of which imagines himself factual." The structure is such that when the first character of each of the letters in the book are placed on a calendar according to their dates, and the individual months are turned sideways, they spell out the subtitle. In addition, the marked dates spell out the word "LETTERS."
Marjorie Godlin Roemer (1987). "The Paradigmatic Mind: John Barth's LETTERS". Twentieth Century Literature (Twentieth Century Literature, Vol. 33, No. 1) 33 (1): 38–50. doi:10.2307/441331. JSTOR441331.