I've come across a handful of textbooks good enough to keep. It's rare, though. First year university, made the mistake of buying all my texts- first years, don't do this! Now, I ask around one year up, see if the books are worth anything at all. If they are it's easy enough to borrow one from someone else how bought it. At the end of the year, if it was actually exceptional, I'll return the borrowed and buy my own copy.