Liest du ein Buch?

This phrase is a direct question used to ask if someone is currently engaged in the act of reading a book. In German, the simple present tense covers both the English present simple and present continuous, meaning this can also mean 'Do you read a book?'

Use this in casual settings when you see someone with a book or want to inquire about their current reading activity. It is a friendly way to start a conversation about literature or hobbies.