What are some important books in NLP

Allen, James F., "Natural Language Understanding", The
   Benjamin/Cummings Publishing Company, Menlo Park, California,
   (Addison-Wesley Publishing Company, Reading, Massachusetts).

   Manning, C. and Schuetze, H. Foundations of Statistical Natural
   Language Processing. Hardcover - 680 pages (July 1999)
   MIT Press; ISBN: 0262133601
   http://www.sultry.arts.usyd.edu.au/fsnlp/promo/

   Jurafsky, D. and Martin, J. Speech and Language Processing.
   http://www.cs.colorado.edu/~martin/slp.html

   Gazdar, G. and Mellish, C., "Natural Language Processing in Lisp:
   An Introduction to Computational Linguistics", Addison-Wesley,
   Reading, Massachusetts, 1989. (There are three different editions
   of the book, one for Lisp, one for Prolog, and one for Pop-11.)

   Michael A. Covington, "Natural Language Processing for Prolog
   Programmers", Prentice-Hall, Englewood Cliffs, NJ, 1994. ISBN
   0-13-629213-5.