Create a config file (e.g “letters”) in tessdata/configs directory – usually /usr/share/tesseract/tessdata/configs
or
/usr/share/tesseract-ocr/tessdata/configs
And add this line to the config file:
tessedit_char_whitelist abcdefghijklmnopqrstuvwxyz
…or maybe [a-z] works. I don’t know. Then call tesseract similar to this:
tesseract input.tif output nobatch letters
That will limit tesseract to recognize only the wanted characters.