Books are, like previously pointed out, more reliable and is authenticated. However with time information becomes obsolete and new editions of books keep emerging in the market, though not always fast enough. Internet makes it possible so that information on webpages like Wikipedia is updated everyday with the occurrence of any changes in the research and the world around us, so especially for students studying realtime problems such as geography and economics the internet helps keeping them up-to-date.
Also, internet provides cheap, vast and various forms of knowledge (videos, photos, articles, etc) which can be a great aide in learning. In fact, many modern schools do not endorse books at all and instead students study from online resources. Of course, the eminent problem is the authentication of sources, for which one needs experience and even teachers can tell you which sites to go to for extra help in their subjects.
In the end, both sources of knowledge are indisposable. If you carry them alongside, like most good things in life, you'll take benefits from both.
