Chapter 10 of the book "Foundations of Computer Science" (http://i.stanford.edu/~ullman/focs.html http://i.stanford.edu/~ullman/focs.html) which is available on

Chapter 10 of the book "Foundations of Computer Science" (http://i.stanford.edu/~ullman/focs.html http://i.stanford.edu/~ullman/focs.html) which is available online provides an excellent introduction to automata theory. After that you can start referring to " Introduction to Automata Theory, Languages, and Computation" by Ullman, Hopcraft and Motwani. This is also the reference book for the Automata course (and taught by Ullman) currently being available in Coursera.