Fundamental theory of neural networks

My friend who's doing a PhD in quantum computation is transitioning towards deep learning because of the inherent toxicity in the field of theoretical physics. He pinged me last night asking for a good reference on a fundamental theory of neural networks. Now I have been reading DNN research here and there for the last decade since I left college but I don't think there's any fundamental theory of neural network as such. I am guessing he needs is an …

computation deep learning deeplearning dnn good networks neural networks phd physics quantum reading reference research theoretical physics theory toxicity