Not on colt starting per se but if you want s coherent more science based overview of behavior training and the interplay of R + and R - then Andrew McLeans work is valuable. I’m not sure why this link just goes to one paper I was trying to link to the general website with books etc.
The really important thing though in all of this is to develop your own feel, timing and presence with the horse. Read widely but never let that interfere with your own intuition.