The e-book is provided in three formats suitable for e-readers:
"Natural Language Processing with Python --- Analyzing Text with the
Natural Language Toolkit" Steven Bird, Ewan Klein, and Edward Loper
Creative Commons Creative Commons Attribution Noncommercial No
Derivative Works 3.0 US License Above link points to online /
downloadable version. O'Reilly also publish a print version.
Very good book on learning the NLTK toolkit and learning Natural