No, I don’t mean building an actual library, though that may be an interesting project to do as an engineer. I mean build a library using all the things you’ve learned in previous semesters. While it’s definitely cheaper to rent used textbooks, or even better, find them online (not that I do that or anything), it might also be worthwhile to invest in buying certain textbooks.
Say there’s a textbook that’s only 50 bucks to buy and it’s actually a good textbook. Not like some of those that are $200 and are filled with stock photos. It might be worth it to buy that textbook to hold onto it. Then, when you’re an engineer later on, you have something to refer to.
The same can be said with all those notes. I made the mistake of throwing away my Calc III notes. I need them next semester for Physics 2, where you need to know how to do surface integrals and whatnot. Now, I have nothing. I’m not screwed though, thanks to Khan Academy, but having notes to look back on as you move into more complicated subjects and classes is always nice to have.
So think about keeping a base of stuff that you might need in the future. Sure, maybe those philosophy notes don’t need to be kept if you’re a mechanical engineer major. But maybe you should hold on to those thermodynamics notes.