Ask colleagues what publishers they are using.
If you have access to course syllabi for classes that require students to read books that are not textbooks, check the publishers represented on these reading lists.
Search online databases to discover who publishes book in your area.
Check directories of book publishers.
Check publisher websites to see what books they are publishing. Search older book titles on Google Scholar to see if they are being cited in the scholarly literature.
From: Writer Beware (Science Fiction and Fantasy Writers of America)