Of course not all old books are worth reading, but to never read a quality old book is to miss something of incalculable worth. C. S. Lewis put it like this:
“There is a strange idea aboard that in every subject the ancient books should be read only by the professionals, and that that the amateur should content himself with the modern books. . . . . This mistaken preference for the modern books and this shyness of the old ones is nowhere more rampant than in theology.
Now this seems to me topsy-turvy. 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. . . . 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 read at least one old one to every three new ones. . . . We all. . . . need the books that will correct the characteristic mistakes of our own period.
And that means old books. . . . We may be sure that the characteristic blindness of the twentieth century. . . . lies where we have never suspected it. . . . None of us can fully escape this blindness. . . . The only palliative is to keep the clean sea breeze of the centuries blowing through our minds, and this can only be done by reading old books.”
C. S. Lewis Introduction to St. Athanasius, The Incarnation of the Word of God.