''An Introduction to Automata Theory, Languages, and Computation'' by Hopcroft, Motwani, and Ullman. 

http://images.amazon.com/images/P/0201441241.01._SCMZZZZZZZ_.jpg

[ISBN 0201441241]

A very approachable and enjoyable book about the foundations of parsing and computing. Guaranteed to provide new and useful ways of looking at common problems.
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CategoryBook