This demonstration goes really well with Michael Nielsen's http://neuralnetworksanddeeplearning.com/ http://neuralnetworksanddeeplearning.com/. At the bottom of

This demonstration goes really well with Michael Nielsen's http://neuralnetworksanddeeplearning.com/ http://neuralnetworksanddeeplearning.com/. At the bottom of the page the author gives a shout out to Nielsen, Bengio, and others. For someone (like me) who's done a bit of reading but not much implementation, this playground is fantastic!