In the Discourse on Method, part 1, Descartes gives a brief autobiography of his intellectuel journey. As he is describing his education he says the following concerning the importance of reading good books.
"I was aware that the reading of all good books is indeed like a conversation with the noblest men of past centuries who were the authors of them, nay a carefully studied conversation, in which they reveal to us none but the best of their thoughts.(p. 84)"
By reading good books from the past, we are not only able to carry on a conversation with these great thinkers, but we are also able to sit at their feet and learn.
C. S. Lewis, in an article entitled "Christian Apologetics", in the book God in the Dock, said, on the same topic, the following, "If one has to choose between reading the new books and reading the old, one must choose the old: not because they are necessarily better but because they contain precisely those truths of which our own age is neglectful. (p. 92)"
In another article, "On the Reading of Old Books", in the same book, Lewis has this to say, "Naturally, since I myself am a writer, I do not wish the ordinary reader to read no modern books. But if he must read only the new or only the old, I would advise him to read the old...A new book is still on its trial and the amateur is not in a position to judge it. It has to be tested against the great body of Christian thought down the ages, adn all its hidden implications (often unsuspected by the author himself) have to be brought to light. Often it cannot be fully understood without the knowledge of a good many other modern books. If you join at eleven o'clock a conversation which began at eight you will often not see the real bearing of what is said...It is a good rule, after reading a new book, never to allow yourself another new one till you have read an old one in between. If that is too much for you, you should at least read one old one to every three new ones. (p. 201-2)"
This is right to the point. A good example of the problem of reading only modern books, can be seen by the discussion between N. T. Wright and John Piper on justification. Each of these authors have written at least one book each, and many articles on the subject (see John Piper, The Future of Justification, and N. T. Wright, Justification). To read just the one or the other is to not have the whole story. Therefore you must read the entire debate to truly understand what is being said. However, read an older book, that has stood the test of time, and the problem is in part, at least, taken care of.