This page says that standardization took place in year ~1700
So, where is it stated that English language has 26 letters and where are all letters defined?
This link https://en.wikipedia.org/wiki/History_of_the_Latin_alphabet says "By the 18th century, the standard Latin alphabet comprised the 26 letters we are familiar with today". But what is the source?
The earliest document advised in answers to this question is
Samuel Johnson's Dictionary of the English Language, published in 1755, had the English alphabet codified.
Also, americans ITA-1 is dated by year 1929, see www.trafficways.org/ascii/ascii.pdf
UPD2: I want something like "there is a book in that library, which is first document which uses modern alphabet", something like Cawdry, Robert.; T. C.","1609", "A table alphabeticall". It is said, that it is available electronically for free under CC0 license, but I am unable to find the text.