In Unicode, a letter is any character with one of the letter categories (Lu, Ll, Lt, Lm, Lo) in the Unicode character database.
There are 52 ASCII letters
There are 117 Latin-1 letters. These are the 115 characters that are members of the Latin-1 char-set:lower-case and char-set:upper-case sets, plus
00AA FEMININE ORDINAL INDICATOR 00BA MASCULINE ORDINAL INDICATOR
(These two letters are considered lower-case by Unicode, but not by SRFI 14.)