Hopcroft, John E., and Jeffrey D. Ullman (2006). Introduction to Automata Theory, Languages, and Computation. 3rd ed Reading, MA: Addison-Wesley. ISBN 978-0321455369 One of the standard references in the field.
Michael Sipser (2006). Introduction to the Theory of Computation 2ed. PWS Publishing. ISBN 0-534-94728-X.
Eitan Gurari (1989). An Introduction to the Theory of Computation. Computer Science Press. ISBN 0-7167-8182-4.
Hein, James L. (1996) Theory of Computation. Sudbury, MA: Jones & Bartlett. ISBN 978-0867204971 A gentle introduction to the field, appropriate for second-year undergraduate computer science students.
Taylor, R. Gregory (1998). Models of Computation and Formal Languages. New York: Oxford University Press. ISBN 978-0195109832 An unusually readable textbook, appropriate for upper-level undergraduates or beginning graduate students.
Lewis, F. D. (2007). Essentials of theoretical computer science A textbook covering the topics of formal languages, automata and grammars. The emphasis appears to be on presenting an overview of the results and their applications rather than providing proofs of the results.
Martin Davis, Ron Sigal, Elaine J. Weyuker, Computability, complexity, and languages: fundamentals of theoretical computer science, 2nd ed., Academic Press, 1994, ISBN 0122063821. Covers a wider range of topics than most other introductory books, including program semantics and quantification theory. Aimed at graduate students.
Books on computability theory from the (wider) mathematical perspective
Hartley Rogers, Jr (1987). Theory of Recursive Functions and Effective Computability, MIT Press. ISBN 0-262-68052-1
S. Barry Cooper (2004). Computability Theory. Chapman and Hall/CRC. ISBN 1-58488-237-9. .
Carl H. Smith, A recursive introduction to the theory of computation, Springer, 1994, ISBN 0387943323. A shorter textbook suitable for graduate students in Computer Science.
Historical perspective
Richard L. Epstein and Walter A. Carnielli (2000). Computability: Computable Functions, Logic, and the Foundations of Mathematics, with Computability: A Timeline (2nd ed.). Wadsworth/Thomson Learning. ISBN 0-534-54644-7. .