I am not a graphics designer, but I can answer the part about you can learn this in books part.
It is possible to learn everything from books alone. Period. Even all the info I learned in university studying for my master's in mechanical engineering could have been learned from books alone. Its possible, but not feasible.
A good education will accelerate the process. It will also open you to a wider world view in a way that you might not otherwise explore. Every profession has lots of convolution, even if its in theory simple. Just because humans have been doing it for a while. So theres always lots of things you should familiarize yourself with.
Remember a true expert knows several alternative methods to accomplish things. This knowledge can really make a huge difference. However, knowing how something is done is not the same thing as knowing how to do it. You still need practice. The school provides practice in a environment that's less serious than work but more demanding than most of us can muster.
On top of this you forget is that the diploma may count arbitrarily for some jobs. Useful or not. So for example if I wanted to work in higher positions it may be mandatory by policy to demonstrate a degree.