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But then again, if you do prefer to have a book in your hand, you can find second-hand books at very low costs on Amazon too.
If you use Google Books and whack in the ISBN number, you can often find a cheap second-hand retailer in the side menu. You can do the same on Amazon, or even use Wikipedia Book Search to manually search with ISBN numbers which will kick back a list of results.
In some cases, you can rip out chapters that you need from Google Books without needing to buy them. Most of the time, pages will be missing, but it is certainly worth a shot.
Always be sure to check the "new", "used" and "collectable" links on Amazon, and do not automatically be drawn to one. A book is a book, and if you want a cheap and shabby copy of the latest edition, that's better than buying an expensive one in a better condition. As long as it's readable, it shouldn't matter too much.
To read more on this, head over to the iGeneration blog.