* The edition: Different editions might have slightly different text, including typos, changes in formatting, etc.
* How you define a "word": Do you include contractions as one word? What about hyphenated words?
That being said, you can get a rough estimate by using a word counting tool. If you're interested in doing that, you can find free online word counters that can analyze text from a file or URL.
Keep in mind that even the most accurate word counter won't be perfect and will likely only give you an approximate answer.