Someone asked me about this:
Q: Aw, crap, I can't believe the intro physics courses have changed textbooks! I was going to buy my friend's old book. Now I have to fork over a whole bunch of money for a new textbook! Do I ABSOLUTELY have to get it?
A: You know what, maybe not. It will depend on the course. I'm taking PHYS 121 this quarter and so I bought the book. Well, you know, it said it was required and all. However, as the quarter progressed and finally came to a close (almost) I realized that I could have gotten along fine with a book from one of the previous quarters, say, that nice white Giancoli text that I can actually lift with one arm without straining my wrist. In fact...
...since the new physics book is the size and the weight of a briefcase filled with cement, I can't take it with me to campus. So, I borrow the reserve copy from Odegaard. Often, however, all the copies are out and I borrow the Giancoli book. Its chapters are almost WORD FOR WORD identical to the chapters in the current text! Not to mention, some things are explained in a slightly less muddled way. Never once did we have homework assigned from the book, it's all online. So, never once did we use the book for any other purpose than reading the material (I also use it to prop up a bookcase and for other various things that require something really heavy). The same stuff (at least as far as Physics 121) may be found in older books. So, if you know someone who doesn't need their textbook from previous quarters and wants to sell it, buy it from them! But maybe wait till the course starts so you can be sure there won't be anything assigned from the book other than reading.