The link below leads to a github repo where you can download the data files for the Text Mining in Practice with R book .  It contains various text related corpora useful for learning and mentioned throughout the book.  The raw text data sources include tweets, emails, movie reviews and other useful files.  I suggest starring the repo instead of cloning since I may be making changes or adding files over time.

https://github.com/kwartler/text_mining

For code and mentions of data sources outside the text mining book check out the blog section of the site.