The profs certainly can't make you get a certain book, but your assignments are often based on the questions in the book and therefore, to have a passing grade,

The profs certainly can't make you get a certain book, but your assignments are often based on the questions in the book and therefore, to have a passing grade, you have to get the book one way or another. Many, if not most, profs in the US will use book assignments instead of writing their own. In addition, the profs may write their own tests, but these also follow the assigned textbook so closely that studying from another book might be a handicap.