A lettered edition is a specific variety of limited edition that is characterized by letters of the alphabet with each book assigned a letter.
Lettered editions are traditionally limited to 26 copies, one for each letter of the alphabet, but it is not uncommon for a lettered edition to be limited to 52 copies with books marked by upper and lower case letters or represented by A to Z followed by AA to ZZ.
These tiny print runs - designed to be collectable from the word go - are a perfect chance for publishers to pull out all the stops. It’s common for these lettered books to be deluxe editions with author and/or illustrator signatures, slip cases, and special features not found in run-of-the-mill editions. When only the rarest modern editions will do, look for a lettered edition.