I generally listen to the podcast "The Rest is Politics" with Alistair Campbell and Rory Stewart. I have just been listening to a long interview by them with Michel Barnier largely, but not entirely, about his experiences during the Brexit negotiations. If you can get a chance to hear it, it is worth your time.

There is a book "La grande illusion: Journal secret du Brexit (2016-2020)" which is, I think, only available in French at the moment, but no doubt an English translation will be produced in time.