How to write a cql query for LOC server ?

Question

How to write a cql query for LOC server ? I am trying to retrieve data from loc server using z39.50 and YAZ dll. Following is the code

query = "Author=" + txtAuthor.Text.Trim() + " and Title=" + txtTitle.Text.Trim() + " and Publisher=" + txtPublisher.Text.Trim();

but i am getting error that the syntax of query is wrong although the textboxes have value. Plz help