I think both bits and bitless bridles for the most part are as harsh as the hands that are using them. A simple snaffle can pop a horses mouth like a nutcracker with hard hands and a bitless bridle can damage a horses nasal bones with too much force. The fit is also just as important and your horses mouth size/shape. Is your horses mouth small, normal or large? Is their palate low? Normal? Is it high? Different bits use different types of pressure and depending on how your horses mouth is one type might be more comfortable ergonomically speaking. In my mind the best bit is the one that my horse is most comfortable in. If you watch your horses body language they will tell you what they like. I have seen a horse go from “hot” to calm simply because they went from poor fitting tack that was causing pain to properly fitted tack allowing the horse to move with ease and without pain. A bitless bridles fit is extremely important as well. The noseband being just 1 inch too low can cause damage simply because the pressure is in the wrong spot. I don’t have any book recommendations, but this is a good study done on bitless bridles that goes to show that they have the same potential to injure and cause pain as a bit in the wrong hands. Also to clarify I am not pro or against bits or bit less. I have had horses that did the best in a Dr. Cooks or a simple hack more. I have horses who prefer a curb and some who just like to keep it simple with a French link snaffle. Like I said above I go with what the horse prefers and sometimes that means trying out multiple bits or bridles until you find what they like best.
https://www.researchgate.net/public..._bridles_and_the_effects_on_equine_locomotion
Edit: If I had to choose one bit my favorite is probably a Myler Comfort Snaffle. My favorite bitless would be both the English and western dr. Cooks.