Katakana is used for "foreign" words, or used to express emphasis. Doesn't your book tell you all this?

There is a useful information also. Please look at this http://www.saiga-jp.com/japanese_language.html

Katakana is used for names, especially people and places. Hiragana can be used to write anything in Japanese, but know that you should always use Katakana for names and Kanji when you are able to. Kanji is used when possible to simplify (hah!) and shorten words into single, complex characters. Furigana is basically small font Hiragana for Kanji. You'll see this at airports or other tourist locations where people might not know Kanji, like children or foreigners. For example: http://www.kanjisite.com/images/furniture/furigana.gif

Wikipedia has some in depth information about the Japanese language system. http://en.wikipedia.org/wiki/Japanese_writing_system I'm not sure if you're supposed to learn to read or speak Japanese first (seems like people have mixed opinions about this). Maybe other Japanese learners could chime in on what was effective for them...?

Japanese has three kinds of letters. They are kanji, hiragana,and katakana. Kanji is ideogram word which is originated in Chinese. Hiragana is phonogram word which is original in Japan. Katakana was created to express foreign words and to express imitative sounds.