Update query not working correctly?

I want to run an update query that will update one column in a table with a value from another column in another table.  Here is the SQL query I am running:
UPDATE tblLetterPrint SET tblLetterPrint.LetterDate =("LTRDATA.LtrDate");
What I am getting is, instead of the VALUE of LtrDate from table LTRDATA, I am instead getting the text string LTRDATA. in the LetterDate field.  What I want to have instead is the column LetterDate, AFTER the query is run, will contain the date value that is stored in the column LtrDate in table LTRDATA.  I am sure there is a small change I am needing to get the above SQL query to work correctly, but I am not seeing what it is...
thank you