I created a book. Colored it and added shadows. Now I duplicated it so i got an exact green duplicate of my original. I have 2 green books.

I went and changed the color of the duplicate to blue, but it changes the color of my original as well. How do I avoid this?

This is presumably because the colour is a shared gradient. If you copy and paste the original, rather than duplicating it, then the copy should have its own gradient, and you can change it without affecting the original.

There's a setting in Inkscape Preferences where you can prevent the sharing of gradients. Inks Prefs > Misc