"Discrete Time Signal Processing" by AlanOppenheim? and RonaldSchafer? (ISBN 0137549202 ). One of the classic introductory textbooks into DigitalSignalProcessing. Now in its second edition. Excellent book. Focuses on elementary DigitalSignalProcessing theory, including discrete signal transforms (DFT, Z-transform, FFT), digital filtering (FIR, IIR), sampling and reconstruction theory, etc.