Aren't you going to be out of a job when computers replace books?
There's lots of answers to this, but I was happy to illustrate my point with a quote a book.
In Douglas Adams' Mostly Harmless, two characters are comparing astrology to the science of astronomy. One of them makes the case that its rules and methods is what gives astrology value, because they serve to bring out the information someone is seeking.
"It's like throwing a handful of fine graphite dust on a piece of paper to see where the hidden indentations are. It lets you see the words that were written on the piece of paper above it that's now been taken away and hidden. The graphite's not important. It's just the means of revealing their indentations."
I immensely enjoy books, and don't think they are going anywhere any time soon, but this question implied that libraries are just book warehouses. In fact, libraries aren't about books at all - we are about information, and access to that information.
Printed and bound books are just one form of "graphite dust" that can be used to reveal the important part - the information they contain. E-books, newspapers, websites, DVDs, journals, mp3s and paintings are also types of delivery media for information.
As long as there information, there will be a need to organize it, convey it, give it context, and help others use it. Talk about job security.