Many letters exist which have not yet been published, but are nevertheless cited in books, articles, essays, etc. Those letters which are confirmed to still be extant, whether in public or private collections, will be given a designation here preceded by XXX. Unpublished letters will be included in correspondence chains, and tagged unpublished.
Those letters which are apocryphal, which is to say they are cited or quoted but are not currently known to exist or have actually existed, are preceded by ZZZ. These letters are included only for reference, and if they are subsequently published will be removed from these tables, renumbered, and the links will be updated. Apocryphal letters will not be included in correspondence chains, and tagged apocryphal.
| Letter # | From | To | Date |
| XXX-0000 | H. P. Lovecraft | Anne Phillips Gamwell | 26-Feb-1925 |
| XXX-0001 | H. P. Lovecraft | Lillian Clark | 6-Jul-1925 |
| XXX-0002 | H. P. Lovecraft | Lillian Clark | 13-Jul-1925 |
| XXX-0003 | H. P. Lovecraft | Lillian Clark | 1-Sep-1925 |
| XXX-0004 | H. P. Lovecraft | Lillian Clark | 2-Apr-1927 |
| XXX-0005 | H. P. Lovecraft | Lillian Clark | 12-Jun-1930 |
| XXX-0006 | H. P. Lovecraft | Lillian Clark | 17-Nov-1924 |
| XXX-0007 | H. P. Lovecraft | Duane W. Rimel | 1-Apr-1936 |
| XXX-0008 | H. P. Lovecraft | Lillian Clark | 18-Sep-1925 |
| XXX-0010 | Robert H. Barlow | August Derleth | 31-Mar-1937 |
| XXX-0011 | H. P. Lovecraft | Duane W. Rimel | 20-Feb-1937 |
| XXX-0012 | H. P. Lovecraft | Hyman Bradofsky | 4-Nov-1936 |
| XXX-0013 | Bernard Austin Dwyer | Clark Ashton Smith | 1937 |
| XXX-0014 | August Derleth | Robert H. Barlow | 21-Mar-1937 |
| XXX-0015 | H. P. Lovecraft | Duane W. Rimel | 20-Jun-1936 |
| XXX-0016 | H. P. Lovecraft | Lillian Clark | 15-Aug-1930 |
| XXX-0017 | H. P. Lovecraft | the Gallomo | Apr 1920 |
| XXX-0018 | H. P. Lovecraft | Lillian Clark | 6-May-1929 |
| XXX-0019 | H. P. Lovecraft | Lillian Clark | 7-May-1929 |
| XXX-0020 | H. P. Lovecraft | Lillian Clark | 20-May-1930 |
| XXX-0021 | H. P. Lovecraft | Richard E. Morse | 24-Dec-1933 |
| XXX-0022 | H. P. Lovecraft | Hyman Bradofsky | 23-Jul-1935 |
| XXX-0023 | H. P. Lovecraft | Lloyd Arthur Eschbach | 3-Apr-1935 |
| XXX-0024 | H. P. Lovecraft | Frank Belknap Long | Mar 1930 |
| XXX-0025 | H. P. Lovecraft | Anne Phillips Gamwell | 29-Mar-1936 |
| XXX-0026 | H. P. Lovecraft | Lillian Clark | 9-Oct-1925 |
| XXX-0027 | H. P. Lovecraft | Maurice W. Moe | 13-Dec-1928 |
| XXX-0029 | Lindsey Tyson | L. Sprague de Camp | Unknown |
| XXX-0030 | H. P. Lovecraft | Lillian Clark | 24-Nov-1924 |
| XXX-0031 | Harold Preece | Tevis Clyde Smith | 16-Dec-1928 |
| XXX-0032 | Harold Preece | Tevis Clyde Smith | 23-Feb-1928 |
| XXX-0033 | Booth Mooney | Tevis Clyde Smith | 1928 |
| XXX-0034 | A. H. Bittner | Robert E. Howard | 2-Jun-1930 |
| XXX-0035 | The Christopher Publishing House | Tevis Clyde Smith | 5-Jan-1932 |
| XXX-0036 | Mrs. F. M. Torbett | Robert E. Howard | 23-Apr-1932 |
| XXX-0037 | Farnsworth Wright | Dr. I. M. Howard | 6-Sep-1936 |
| XXX-0038 | Desmond Hall | Robert E. Howard | 16-Nov-1933 |
| XXX-0039 | August Derleth | Robert E. Howard | 8-Jun-1935 |
| XXX-0041 | Harry Bates | Clark Ashton Smith | 22-Jan-1932 |
| XXX-0042 | Benjamin De Casseres | Clark Ashton Smith | 16-Jun-1933 |
| XXX-0043 | Farnsworth Wright | Clark Ashton Smith | 29-Sep-1933 |
| XXX-0044 | Clark Ashton Smith | August Derleth | 2-Apr-1937 |
| XXX-0045 | Clark Ashton Smith | August Derleth | 24-Feb-1932 |
| XXX-0046 | Clark Ashton Smith | Samuel Loveman | 22-Apr-1915 |
| XXX-0047 | Clark Ashton Smith | Donald Wandrei | 4-Aug-1925 |
| XXX-0048 | Clark Ashton Smith | Samuel Loveman | 6-Apr-1918 |
| XXX-0049 | H. P. Lovecraft | Lillian Clark | 29-May-1928 |
| ZZZ-0000 | H. P. Lovecraft | Jacques Bergier | Unknown |