Easy enough to do.
Just click the button at the top of the page that says "Search" (not the textbox with the search button next to it) which will take you to the Advanced search page. There you can do a search by username and you can even limit the search to specific genres like Romance or Sci-fi/Fantasy if you are looking for specific types of books.
From the FAQ in the Help Desk: Is it possible to search for books by a particular OP?
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