Here's another fine quote from Thoreau (this time from A Week on the Concord and Merrimack Rivers):
Certainly, we do not need to be soothed and entertained always like children. He who resorts to the easy novel, because he is languid, does no better than if he took a nap. The front aspect of great thoughts can only be enjoyed by those who stand on the side, whence they arrive. Books, not which afford us a cowering enjoyment, but in which each thought is of unusual daring; such as an idle man cannot read, and a timid one would not be entertained by, which even make us dangerous to existing institutions -- such I call good books.