Counts of letters and words have been made from various bodies of text over the years. Perhaps the earliest well-known corpus was the assembled by W.N. Francis and H. Kucera of Brown University (hence known as the Brown corpus). It contains one million words of American English from 1961 in 15 categories of 500 texts. If it has a use today, it is because matching studies were done of British English and Indiana English among others.
"Letter and Word Frequencies,"
Word Ways: Vol. 39
, Article 13.
Available at: https://digitalcommons.butler.edu/wordways/vol39/iss2/13