Amazon takes a minimum of 1 calendar day to get a paper book to me, and that's at some extra cost. I can get a paper book from my local library in under an hour if it's on the shelves. (It helps that I live a block away from the Brooklyn Central Library.) If the book I want is not on the shelves, them amazon may be quicker; and I may order from them if I really need the book ASAP. For ebooks it's a wash on time, though amazon does have a much better selection of ebooks.
As with all advice, adjust to fit your personal circumstances. If you live 2 hours away from the nearest library, you may be willing to trade money for time, especially if it['s a book you're pretty sure you're going to want to refer to repeatedly, rather than just read once and put on a shelf, never to be looked at again. I suspect most folks live closer than that. Many people who live in an urban or suburban area walk or drive by a public library on an almost daily basis.
There are certainly reasons to buy some books rather than merely borrowing them. But my main point is that before reflexively going to amazon and clicking "buy It now", you should take a few seconds to consider whether you might be better off searching for it at the library first. I am astonished at how many book hoarders I know who have apartments stacked with paper tomes, sometimes almost dangerously so, who somehow never consider setting foot in a public library.
Thus spake Eliezer:
It seems that many here might have outlandish ideas for ways of improving our lives. For instance, a recent post advocated installing really bright lights as a way to boost alertness and productivity. We should not adopt such hacks into our dogma until we're pretty sure they work; however, one way of knowing whether a crazy idea works is to try implementing it, and you may have more ideas than you're planning to implement.
So: please post all such lifehack ideas! Even if you haven't tried them, even if they seem unlikely to work. Post them separately, unless some other way would be more appropriate. If you've tried some idea and it hasn't worked, it would be useful to post that too.