Perhaps the best introductory book written on the fundamentals of digital circuits, "Bebop to the Boolean Boogie," Second Edition, by Clive "Max" Maxfield, is must reading if you are getting into the digital electronics field. It is very informative, understandable and downright enjoyable. (Newnes, an imprint of Elsevier Science, 2003, ISBN 0-7506-7543-8)