I think nltk in combination with another reference is a great way to learn. You can start out with a small project, have it functional off the ground, then dig deeper.

http://ocw.mit.edu/courses/electrical-engineering-and-comput... is excellent as is http://metaoptimize.com/qa/a stackoverflow style site which is active albeit specialized.

Perhaps try this instead? http://nlp.stanford.edu/fsnlp/

Yeah, but all theory is included :-)