Incognito: The Secret Lives of the Brain
"...Third, our brains are composed of vast, complex, and shifting collections of subparts, most of which we have no access to; this book was written over the course of a few years by several different people, all of whom were named David Eagleman, but who were somewhat different with each passing hour."

A succinct summary of the book from the afterword. Free will advocates will have to do some pretty fancy footwork to keep up.