For more theory on articles, you can have a look at these pages:

http://owl.english.purdue.edu/owl/resource/540/01/

http://www.englisch-hilfen.de/en/grammar/artikel.htm
http://www.englisch-hilfen.de/en/grammar/artikel2.htm
http://www.learnenglish.de/grammar/articlestext.htm
http://www.unc.edu/depts/wcweb/handouts/articles.html

If you'd like to revise Unit 2 a bit, you can use the Unit 2 test from the book (together with key) below: