These are the books I have found which make the case for simple design. I will add more as I find them.
Simplicity: A Matter of Design, Per Mollerup.
Simple and Usable: Web, mobile, and interaction design, Giles Colborne.
Insanely Simple: The Obsession That Drives Apple's Success, Ken Segall.
The Simplicity Shift, Scott Jenson.
The Laws of Simplicity, John Maeda.