General Python programming books:

• Dive into Python by Mark Pilgrim. Available on the Internet from http://diveintopython.org/ This is a good introductory book
• Python Cookbook by Alex Martelli. Shows good code examples. Not so good as an introductory book

Specialized books:

• Programming  Collective Intelligence by Toby Segaran. About machine learning on the web 2.0 with examples in Python. This book is recommendable if you want to do topic mining in texts.
• Natural Language Processing with Python by Steven Bird et al. Describes the NLTK Python package.  Also available from http://www.nltk.org/book