"which" can be used when there are multiple options. That is how it used in that sentence.
Which shirt should I wear? (There is more than one shirt.)
Which book should I read first? (There is more than one book to consider.)
Which journalistic style did he use? (There is more than one style he may have used.)
Depending on which shirt you wear, you may need to wear a sweater to stay warm.
Depending on which book you want to read first, you may want to wait until you have more free time before you begin reading it.
Depending on which journalistic style...
However, it's very common (for American English speakers at least) to use "what" instead of which.
What book should I read first?
What shirt should I wear?
What lipstick looks better on me?