Both are different media.
Books usually contain text and images only.
But internet contains text,images, videos, sounds ,etc.
Most important feature of internet is interactivity.
it makes internet unique.
For reading long text contents printed media is the best even though it is possible to read on net.
Especially for literature like novels, books are best, i think.
May be that's why internet (websites) provide facility to print the content.
That means even internet agrees the importance of printed media.
Books can't replace internet. But internet can replace books in some way.
This doesn't mean that internet is always best, rather, books are best in some cases.
Another point is that, a book can be read on internet (either on the screen or on paper after printing it) but we can't access the internet from a book :)
So, internet for interactivity and books (printed media) for reading.
This is what i think.