Also, this book, The Second Brain, is a bit older, but relevant:

https://www.amazon.com/Second-Brain-Groundbreaking-Understan...

The gut is a second brain, kinda...I have always been fascinated by this book:

The Second Brain

https://www.amazon.com/Second-Brain-Groundbreaking-Understan...

Author is Michael D. Gershon M.D., chairman of the Department of Anatomy and Cell Biology at Columbia University's College of Physicians and Surgeons at the Columbia-Presbyterian Medical Center in New York City.

There is a really good book written in 1999 that expounds on your research doctors point (and is written with a very approachable tone)

http://www.amazon.com/Second-Brain-Groundbreaking-Understand...