If you really know what you're doing, you can just not buy the book at all, or substitute it for a better, cheaper book. Somewhere around multivariable calculu

If you really know what you're doing, you can just not buy the book at all, or substitute it for a better, cheaper book. Somewhere around multivariable calculus, the math/CS building at my college was being remodeled, and professors were dumping books they no longer wanted into a pile in the hall. I was having a challenging time with some of the calculus material, so I rooted around the free books until I found one that I could understand better. The exercises were all the same basic sorts of things, so ultimately it really didn't matter which book I used, just so long as I got through the material. The professor asked me later why I wasn't writing down the exercise numbers he was assigning as homework any more, and I explained I was using a different book. He didn't seem surprised.