I have a 22 year history of working with natural language processing, but for the most part this was a low level of effort (perhaps averaging 3 to 5 weeks a year). For learning (and perhaps for some production work if you extract the parts that you need) I can very much recommend NLTK.

NLTK developers (or aggregators since NLTK is an aggregate of smaller projects, with a lot of new work added) Steven Bird, Ewan Klein, and Edward Loper are writing a complete book on NLP using NLTK that looks good. I wish that I had a good resource like this 22 years ago!