In textbook publishing, it's very important to put out a new edition every year or two. Otherwise your sales will disappear, since everyone will buy the book used. For a book like Halliday and Resnick that's been around since the 1960s, it doesn't take a very large number of people selling theirs to satisfy the demand of all current physics students.
So you put out a new edition in which you shuffle all the exercises so that students can't do their homework, and you have someone mess with the text and the formatting to make it look like a real change. You add glossy pictures, because you get a much bigger visual impact from changing the pictures than from actually changing content.
For an old book, this is a problem because often the authors are dead or retired, or think the book is just fine. Now you have to find someone who would like his name added to a classic text who will sign off on the job. Today it's Halliday, Resnick, and Crane. For Arfken's old mathematical methods for physics text, it was Weber.
So you put out a new edition in which you shuffle all the exercises so that students can't do their homework, and you have someone mess with the text and the formatting to make it look like a real change. You add glossy pictures, because you get a much bigger visual impact from changing the pictures than from actually changing content.
For an old book, this is a problem because often the authors are dead or retired, or think the book is just fine. Now you have to find someone who would like his name added to a classic text who will sign off on the job. Today it's Halliday, Resnick, and Crane. For Arfken's old mathematical methods for physics text, it was Weber.