I held onto my books to have as references. Of course, this was back when the ubiquitous Internet and "instant access to everything" was some years away.
Why should I be reduced after leaving an education, to solely what I can remember in my head and what my current circumstances keep refreshed in my mind and memory? Education was an enormous investment not just of money but also of time and effort. It makes sense to me to keep the resources I used at hand; it helps keep my access to that education at hand.
Seen that way, while book prices are ridiculous, that expense as compared to the entire expense not just in money but in time and effort, is relatively small. It's an investment I've made against the future, where the best recollection and use of my education may be paramount.
And... Those prices are simply unfair. For a system that purports to provide its students tools for life -- not just formulae, but ways of thinking and being. To then increase the burden on those students of taking with them the references they used and might wish to retain as part of retaining that education... It seems that their actions in this matter oppose their rhetoric.
Then again, even when prices weren't so high, I saw a lot of my fellow students simply discarding their books -- at the end of semester, not just after graduation. Many people do "move on" and live "in the present."
Except, those people then seem to be the ones who can't recall what they learned, and who don't do as thorough and good a job in their work.
Living in the present, without the full benefit of history. And reacting based upon a more or less vague impression, rather than detailed knowledge.
What, really, do our institutions want us to retain? How do they want us to live our lives. As demonstrated through their actions -- e.g. these book prices -- rather than their rhetoric?
Anyway...
As for electronic copies. I find I read a lot better from printed pages than from screen. And, as others have pointed out, the "legitimate" electronic versions all too often come with electronic leases -- leashes -- with access deniable purposefully or inadvertently at any point in the future.
My Discrete Math book is still as accessible to me, today -- with any particular personal notes of interest and focus I may have added -- as it was when I bought it. I had to pay for it, and I've had to schlep it around. But I've also had to do that with my education, taking up premium space in my gray matter. I paid a lot more for the education; I hope and believe keeping the book handy helps me retain that larger value.
P.S. And I still find that the good books provide me more useful and valuable access to their information than an ever more cluttered and noisy Internet -- for all that their are many gems -- resources and contributors -- on it. These days, if you can only find them.