[Yreval]

This book is a good practical introduction that walks you through the basic ideas as you develop some basic functionality. http://neuralnetworksanddeeplearning.com/

I'm often pretty skeptical of e-books and self publications, but the above link is pretty good (and the video series linked here references it as well.) The Goodfellow book that another commenter mentioned is a high-quality survey of the field and a nice, high-level overview of different research directions in deep learning, but isn't as pragmatic as an introduction.