Read men, as well as books, and your own heart, in order that you may read others. Copyists, you know, are generally bunglers. There is nothing that equals what is immediately from the life. We need always be making our observations, wherever we are, or wherever we go. If we get a system of human nature, or experience, or any thing else, from books rather than from our own knowledge, it will be liable to two disadvantages. First, it is not likely to be so near the truth; for systems which go through several hands are like successive copies of a painting, every copy of the preceding one is more unlike the original--or like the telling of a tale, the circumstances of which you do not know of your own personal knowledge: every time it is repeated there is some variation, and thus it becomes further removed from the truth. Thus Agrippa showed his wisdom, when, instead of depending on the testimony of others, he determined to hear Paul himself. Secondly, if it be correct, still it will not be so serviceable to you as if it were a system of your own working. Saul's armor might be better than David's sling; but not to him, seeing he had not proved it.
What's Fuller getting at in this quote? As helpful as books may be, if we spend too much time in them, it might begin to distort reality. We must live life, attend to our own hearts and work at the application of truth ourselves. There are so many applications here from not preaching someone else's sermons or relying too heavily on commentaries to failing to apply and have a tender heart. It's often been my experience that those pastors who lay the heavy load on people aren't too great at applying things in their own heart. Those sermons with the ripest application for my hearers tend to be when I have worked hard to apply this to my own heart first.
I commend this sermon to you for reading again. I have a PDF I'd be happy to email you if you click on the link at the top right of my blog. Thanks.