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.
We may be sure that the characteristic blindness of the twentieth century—the blindness about which posterity will ask, "But how could they have thought that?"—lies where we have never suspected it, and concerns something about which there is untroubled agreement between Hitler and President Roosevelt or between Mr. H.G. Wells and Karl Barth. None of us can fully escape this blindness, but we shall certainly increase it, and weaken our guard against it, if we read only modern books. Where they are true they will give us truths which we half knew already. Where they are false they will aggravate the error with which we are already dangerously ill. The only palliative is to keep the clean sea breeze of the centuries blowing through our minds, and this can be done only by reading old books. Not, of course, that there is any magic about the past. People were no cleverer then than they are now; they made as many mistakes as we. But not the same mistakes.
For my own part I tend to find the doctrinal books often more helpful in devotion than the devotional books, and I rather suspect that the same experience may await many others. I believe that many who find that "nothing happens" when they sit down, or kneel down, to a book of devotion, would find that the heart sings unbidden while they are working their way through a tough bit of theology with a pipe in their teeth and a pencil in their hand.
No doubt some of you immediately recognized the above quotes as coming from C.S. Lewis's introduction to Athanasius' On the Incarnation. CSL's wise and witty endorsement is a wonderful bonus to what I know will be a rewarding book—one which I'm about to tackle. Though not with a pipe in my teeth.