One throws at the edges of the books so the knives can penetrate between the pages. I have not tried this idea, but it sounds good. It is probably impractical to hang a thick collection of phone books on the wall, but if you have something convenient to mount them on, it would probably work. Most knife throwing takes place outside. We assume that wood chips litering the ground is not a problem.
Wood makes the best target primarily for aesthetic reasons.
Wood is also more flexible in terms of the size and shape of the target. Wood of the sort required isn't always or everywhere available, and it may not be inexpensive. The traditional wooden target is a burl, a section of a tree trunk. Ideally, they should be as wide as possible, even up to three feet or more in diameter, and cut as much as two feet or more thick. Assuming you have to tools to cut this stump yourself, you can resurface your target when it needs it with another cut, until the whole is too thin to be useful.