It's here: https://github.com/certik/theoretical-physics/, I was hoping more people would contribute to the effort, but so far I didn't manage to spark enough interest. It's open source, it's out there and if I find at least a single person willing to contribute to get it polished, the development will pick up.

In the absence of it, I would have to drive it hard as my main effort, but right now my main effort is LFortran/LPython, we are making excellent progress there, so I am not spreading too thin until the compilers are delivered.

If anyone is interested in the theoretical physics book, please let me know!