In the forward, he puts forth a good motivation for the book. There's a wide disconnect between the various versions of Jesus we might encounter from scholarship.
The gap between the "historical Jesus" and the "Christ of faith" grew wider and the two visibly fell apart. But what can faith in Jesus as the Christ possibly mean, in Jesus as the Son of hte living God, if the man Jesus was so completely different from the picture that the Evangelists painted of him and that the Church, on the evidence of the Gospels, takes as the basis of her preaching?
As historical-critical scholarship advanced, it led to finer and finer distinctions between layers of tradition in the Gospels, beneath which the real object of faith - the figure of Jesus - became increasingly obscured and blurred. At the same time, though, the reconstructions of Jesus (who could only be discovered by going behind the traditions and sources used by the Evangelists) became more and more incompatible with one another: at one end of the spectrum, Jesus was the anti-Roman revolutionary working - though finally failing - to overthrow the ruling powers; at the other end, he was the meek moral teacher who approves everything and unaccountably comes to grief. If you read a number of these reconstructions one after the other, you see at once that far from uncovering an icon that has become obscured over time, they are much more like photographs of their authors and the ideals they hold. SInce then there has been growing skepticism about these portrayals of Jesus, but the figure of Jesus himself has for that very reason receded even further into the distance.
All these attempts have produced a common result: the impression that we have very little certain knowledge of Jesus and that only at a later stage did faith in his divinity shape the image we have of him. This impression has by now penetrated deeply into the minds of the Christian people at large. This is a dramatic situation for faith, because its point of reference is being placed in doubt: Intimate friendship with Jesus, on which everything depends, is in danger of clutching at thin air.
Pope Benedict makes the point in the third paragraph there that if we are led into these various theories, we lose sight of Jesus as our Savior. We don't think we have much knowledge of Him at all. We spend time pondering all the different versions of Jesus, and I myself feel a lot more comfortable with the version of the 60's era hippie who is more than happy to let me do what I want. That isn't Jesus though.
Time and time again, the people of Israel found something "better" than God. This usually took the form of other gods, which they followed and worshipped and which led them away from the true God who created the Universe and all that is in it. I find it is still easy to do this, especially with our new "gods" of atheism telling us so many lies about ourselves, our world, and our Creator. I hate to be a conspiracy theorist, but where do these lies come from? Maybe the father of lies.
Go pick up your Bible and read one of the Gospels. Don't just skim through it because you've heard it over and over again at Mass. Read it anew. Don't try to put Jesus in some 21st century mold. He's God. Get to know Him. I highly recommend this book as well for that purpose.