The book "Neural Networks and Deep Learning" written by Michael Nielsen locate at: http://neuralnetworksanddeeplearning.com
I read a lot of papers and books about machine learning, most of them are quite heavily into mathematics, except this one.
It's not without mathematics, on the contrary, it taught me a lot of mathematical knowledge.
I like the writing style of this book, while now I have finished reading this book, donate a bit to the author. Hope he can keep writing :D