This is the greatest online Deep Learning book! Michael A. Nielsen does a nice job to bring nerd (like me :->) to the filed of Deep learning. There are lots of interactive programs to teach you what is really behind all the algorithm and give you intuitive knowledge of all of them instead of using lots of mathematical work to let you learn. Also, the author lead us to write python program from scratch and achieve pleasurable result.
I strongly suggest people who want to learn Deep Learning read this book first.