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