A large part of the interest and scholarship involving weird fiction has focused on the correspondence of different authors, most notable H. P. Lovecraft, who was a phenomenal epistolarian, and who corresponded with many other weird fiction writers such as Clark Ashton Smith, Robert E. Howard, August Derleth, etc. Many of their letters have been archived in university libraries, or else saved and sold among collectors, and a subset of these have been published in various books and journals - Lovecraft's published letters and postcards occupy well over a dozen volumes, and still only represent a fraction of those he sent over his lifetime, but enough remain and have been published to give an intriguing look at the exchange of ideas, congratulations, and commiserations between various authors of weird fiction, as well as important information of interest to biographers and bibliographers.
For the purposes of this wiki, each published letter or postcard is given a seven-digit number code. The number code is used solely to uniquely identify each letter so that it may receive it's own page to show where it has been published or referenced. The first three digits of the code correspond to the volume the letter is from (or, where it has been published in multiple volumes, the primary reference publication), and the last four digits indicate which letter in the volume it is. For example, 000 is the three-letter code for the Selected Letters of H. P. Lovecraft (Arkham House, five vols.); the code "000-0101" would refer to letter #101 in the SL, and its wiki page is 000-0101.
Each source will have a list of letters identifying sender("From"), receiver("To"), and date mailed ("Date"), as well as the identifying number ("Identifier"), and each individual letter will be sorted into categories based on sender and receiver. A master spreadsheet for all letters and identifiers on the wiki is available here: Wikithulhu Correspondence Index
Note on Dates & Abridgement
Many letters are abridged, and not always in the same way; where possible difference between abridgements will be notes in the summary of the letter. Some letters are noted as "unabridged"; this is to distinguish them from abridged versions of the same letter, but does not always mean that the letter is complete - only that it is produced unabridged from its source, which may include copies or transcripts. Likewise, earlier and later sources do no always agree on what date a letter was mailed, the date for the later source is presumed to be more correct and will be listed, but in the letter summary the discrepancy will be noted.