is used both as a demonstrative adjective as well as an interrogative adjective.
Demonstrative adjectives are used to indicate specific items in relation to ourselves. A demonstrative adjective is used to specify a particular instance or set of instances of a noun. The four demonstrative adjectives in the English language are this, that, these and those, and are placed before the noun being demonstrated. The interrogative adjectives (which and what) modify nouns and are used in questions. It is same as interrogative pronouns except the fact that they cannot stand on their own.
Few instances on both the usages would help you understand the difference between the two:
Which guidebook should be followed?
Which of these should I wear tonight?
Which book were you referring to?
Which of them are your parents?
The book, which was borrowed by her, was returned to me last evening