It depends on your long term plans/goals. Assuming that you have aspirations beyond the three books, I would save your personal branded domain for your blog, bio and other information. Use your other domain name for the books. Of course you can link back and forth between them. If the books take off and you want to keep conversations about the books separate from other stuff, it would be hard to do on your own domain name down the road. Though it is a pain to develop and maintain multiple sites, if I was writing books, I would want them to be on their own domain.

I spent 6 years marketing myself as a website designer, then focused more on marketing myself as a photographer for about 5 years and then switched back to focusing on web again. This has been an easier process because I had individual sites for both web design and photography. My own personal domain name was always kept separate and is a collection of my ambitions and life over the years.

In the end, do what you feel best represents what you are about and how you feel the books will be best marketed.