I second the recommendation for Lou Burroughs' book. Two other excellent, and more current ones, are "Microphone Engineering Handbook" by Michael Gayford and the "American Institute of Physics Handbook of Condenser Microphones" by Wong and Embleton. Unfortunately Gayford is out of print and the AIP book is only about omnidirectional (pressure, measurement) mics, but both contain solid information that can be found in print nowhere else. Another excellent reference is "Microphones" by A. E. Robertson, originally a BBC training manual and reprinted by at least two publishers (also long out of print).