I have occasionally bought a textbook that had lasting value. And with all the work involved in understanding the contents, I hate to get rid of them. I even, a couple of months ago, came across one of my father's old textbooks, a book of mathematical tables (base-10 logarithms, etc.), plus some interesting formulas. Not too useful now, perhaps, but interesting.
Now I'm starting to see *vanishing* textbooks. I recently downloaded some beta-version software (fortunately free of charge) from Microsoft, but after a while of reading the documentation bumped up against a message to the effect that that version had expired and I had to get a new version (again free of charge). It's a way to try to ensure that I don't spend lots of time reading out-of-date stuff, but having to update frequently could be a bit of a nuisance. Students can often buy (at cheap prices) computer software that expires at the end of the semester. That's an extra incentive to finish all the projects on time, since you know the tools will disappear! Calculators probably have basic enough functions that it's not too important to stay up to the minute with them.
-- Vincent Johns Please feel free to quote anything I say here.