Assessing the relevance of textbooks
To determine the relevance of a textbook for future reading, you have to extrapolate your past field of study and interests, based upon which you bought the textbooks in the first place.

If you intend to remain in your current field, keep the books immediately pertinent for that discipline. Otherwise, keep mainly the textbooks on reusable skills, such as math and statistics, computer programming, foreign languages, etc.

You may also want to ask senior students/researchers what books they kept on their shelves.