Bird, Steven, Ewan Klein and Edward Loper (2009) Natural Language Processing with Python: Analyzing Text with the Natural Language Toolkit, O'Reilly Media, http://www.nltk.org/book, Chap. 10.
Jurafsky, Daniel and James H. Martin (2009) Speech and Language Processing: an Introduction to Natural Language Processing, Computational Linguistics, and Speech Recognition, Second Edition, Pearson Education International, Chapters 17-21