I found, ヒロ and キレイ (in the magazine "Fine Boys"), are interesting examples of the uses of katakana in Japanese. The first was Muzushima Hiro's name, his surname written in kanji and his given name in katakana. This is an example of a common trend in Japan today, to use katakana to write Japanese names and words to draw emphasis and "be cool", as it is popular to buy Western and foreign things and use foreign words.
The second word is actualy a Japanese word, but it was written in katakana to draw emphasis to the fact that the shirt is "きれり". In a sentence of all kanji and hiragana, a word in katakana would stand out and draw the reader's eye to it.
I think each textbook has a different way of explaining the uses of katakana becuase it is very difficult to explain a grammatical function as well as a cultural phenomenom. But, they all said essentially the same thing, that katakana are used for onomatopoeia, emphasis, and loanwords form foreign languages. These are all the obvious uses of katakana, but there are more, subtle, pervading uses of katakana in Japanese language and society, such as cultural trends and ideals.
