I believe that printed books are way much better than books on CD. In my opinion printed books have many advantages. You can use it any time you want not just when you have a computer, you can put notes there.
On the other hand books on CD take up little space. But anyway you cant use them whenever you want, for example in the subway or in the park...
Finally I can say that it is better to have both ways to use books. The most important thing is the willing to read!