Here are some ideas: First, I expect students to read the textbook.

I don't think the students really got anything out of them if they hadn't read the textbook.

You have to read the textbook.

I could've just read the textbook.'

Gone are the days where the occasional lazy teacher could simply kick back and get students to read the textbook.

If it was only about the content, the instructor would just stand at the front of the room and read the textbook to you.

I have a new message that might not be very popular: Read the textbook.

If I just go straight to the clicker questions, it is clear they should be reading the textbook.