The following are just a few resources for learning more about ethos ...
Read more about ethos on the Purdue Online Writing Lab's page on the rhetorical situation.
Wikipedia's ethos page is also a good starting place for reserach, especially if you want to quickly learn more about other elements of rhetorical study.
This Brigham Young University page on ethos also links to a variety of related rhetorical concepts.
The Encyclopedia Brittanica definition offers some more historical nuance, if you're interested in further study.
The International Encyclopedia of Communication's ethos entry shows how ethos is taken up in more than just one discipline.