I kept every textbook I ever bought at university, save my Sociology 150 and Poli Sci 106 texts, because they were shite. My Poli Sci prof had the coolest name ever though... James Busumtwi-Sam. So much fun to say...
I kept them all because I found myself referencing them when I tried to write papers. I'd think, "Oh, I studied this in x, let me go get that text". Plus it is cool to look at my bookshelf and see my entire undergraduate degree sitting there. It helps that I'm interested in my field though, so I still find myself going back to my texts, primarily the upper level theory ones and my environmental criminology book, because WOW, people are fucking RETARDS when it comes to crime and criminal justice issues.