Dear native English speakers,
which would sound correct?
(a) The Encyclopedia Britannica is probably the best known work for English-speaking people. It first appeared in Scotland.
(b) The Encyclopedia Britannica is probably the best known work for English-speaking peoples. It first appeared in Scotland.
Definitely the first one. "Peoples" is the plural of the singular noun "people", which is defined as
http://www.dictionary.com/browse/people?s=t said:
the entire body of persons who constitute a community, tribe, nation, or other group by virtue of a common culture, history, religion, or the like: the people of Australia; the Jewish people.
In other words, b) would be like saying the encyclopedia is only for people from English-speaking countries, whereas a) includes all people who speak English, regardless of where they live.