answer:Depends on the subject and the focus of the book. A fast-changing subject dealt with by a book whose focus is to capture the latest info could have significant differences. A slower-paced or core-focused book could see largely cosmetic (page number) or clarification/example changes with perhaps only a chapter or handful of paragraphs being added as new or updated content. Same goes for the course, a course focused on the most contemporary advancements tends to make older editions irrelevant faster. When in doubt ask the professor, just make sure their name isn’t on the book first. If it’s just a chapter or some minor updates, attending lecture, keeping up on problem sets, and doing a little online research can frequently make up the differences between this year’s and previous editions. (Assuming the last edition is at least somewhat frequent of course. Even slow subjects can see major updates over a decade).