Garey MR and Johnson DS, Computers and Intractability: A Guide to the Theory of NP-Completeness, vol A series of books in the mathematical sciences (W H Freeman 1979)
Harry R. Lewis, Elements of the Theory of Computation (Prentice-Hall 1998)
Hopcroft JohnE, Motwani R and Ullman JD, Introduction to Automata Theory, Languages, and Computation (Pearson Education 2003)
Rayward-Smith VJ, A First Course in Computability, vol Computer science texts (Blackwell Scientific 1986)
Sipser M, Introduction to the Theory of Computation (3rd ed., International ed, Cengage Learning 2013)