Neural Networks and Deep Learning is a free online book. The book will teach you about:

Neural networks, a beautiful biologically-inspired programming paradigm which enables a computer to learn from observational data

Deep learning, a powerful set of techniques for learning in neural networks

Neural networks and deep learning currently provide the best solutions to many problems in image recognition, speech recognition, and natural language processing. This book will teach you many of the core concepts behind neural networks and deep learning.

For more details about the approach taken in the book, see here. Or you can jump directly to Chapter 1 and get started.