Many books are in the public domain now and can be found online for free. Here's a good site that even has books in different formats like Kindle as well as plain text:
The Hound of the Baskervilles!
Recently I've done some software testing and having large amounts of text available to copy/paste has been useful too. If you want to count text characters I would recommend using Notepad++ as it is accurate, MS Word isn't for example.