Beginning with an informal introduction to language equations, this book presents a framework for a general theory for solving systems of equations and relations between languages. Classical language equations, generalized derivatives, Boolean language equations, and implicit equations are presented systematically. An exploration of mixed systems and open problems rounds out the presentation.
If you dig regular languages, this is absolutely outstanding and a fine extension to Sipser's Theory of Computation or Ullman/Hopcroft's An Introduction to Automata Theory, Languages and Computation. Required reading for all automata researchers and bioinformaticians. I don't know any other books that bring so many magnificent results concerning language equations into one snug place.