Speak from Your Gut
judithweinman.medium.com
judithweinman.medium.com
But the subject is important and worth studying from various perspectives viz. Yoga(Kanda)/Qigong(Dantian)/Zen(Hara)/Modern Science(nerve plexus). The following two books are a good place to start from;
1) The Second Brain by Michael Gershon.
2) Hara : The Vital Center of Man by Karlfried Durckheim.