D. Jurasfky and J. Martin, Speech and Language Processing Prentice-Hall (2008). It should be available from Waterstone's on campus; a few volumes are also on reserve in the library.
Steven Bird, Ewan Klein, and Edward Loper Natural Language Processing with Python – Analyzing Text with the Natural Language Toolkit. O'Reilly (2009).
Christopher D. Manning and Hinrich Schutze, Foundations of Statistical Natural Language Processing, 1999. ISBN 0-262-13360-1