To maximize your company's success, you need to continually feed your brain with new strategies and business practices. And many of us have found this nourishment in business books. But over time, as your business passes through its stages of maturity, the books that once fueled your motivation lose their relevancy. Now their singular purpose seems to be filling the empty gaps on your book shelf. Here we took a look at three companies—BookMooch, PaperBack Swap and Half.com—that offer an online community where you can trade-in your useless business books for ones that may offer more apt advice.