I would recommend the Fundamentals of Government Information: mining, finding, evaluating, and using government resources text after reading the Locating U.S. Government Information Handbook because the former builds upon the introductory information found in the latter. 
While a little dated, the Locating U.S. Government Information Handbook text offers an introduction into most government websites and provides search strategies into finding government documents. This text will help you get orientated to government documents.
