Thomas Byttebier wrote down some of his thoughts on copywriting.

... copywriting is an often overlooked aspect of UI design. That’s a serious shortcoming, because if you come to think of it: most of an app’s user interface simply is text. Hide the icons, imagery or colors and every app or web site is basically just letters and numbers. There may be some data in there, a significant part of it is UI. It would be stupid not to take excellent care of it.

Be sure to also check out his posts on icons and typography.