A simple search in Summon, as shown below, will find books and articles by a particular author, but also books and articles about that author, or which only mention him or her in passing:
But what if you are only interested in works by Barthes? There are two options, firstly, you can click on his name in any search result, to be taken to a list of those of his works included in Summon:
Alternatively, you can go to Summon's advanced search screen. If you are starting from the library homepage, follow this link:
Or from Summon's results screen, follow the Options link at the right of the search box:
Once on the advanced search screen, change the first All Fields box to Author, and run your search again:
For living authors, use a general search engine to see if they have a homepage or online list of publications, perhaps on the website of a university or other institution to which they are attached. if that institution maintains a repository, that can be a worthwhile place to search too.