I think that was a great letter to write your wife!
I absolutely hate it when JWs say that "it is the closest to the truth that is out there" or they say "if we leave, where else is there to go?" Like you said, it is a cult and those phrases seem to be somehow implanted in our brain at some point.
Since your wife is open-minded and seems to want to do research, I would encourage her to research the basic history of the watchtower society. How did it begin? Where did Russel recieve his influences? What exactly was the "great apostacy" that happened after Russel died?
If she does her own research, she will find that the witnesses today are so far removed from what the religion once was. The question she will inevitably be searching for is ... when exactly did it become "the truth"? I think it is especially important to do this research if you were raised in the organization. You don't know where you're going unless you know where you've been.
What was especially helpful to me was when I researched the off-shoots of Watchtower society. Believe or not, several of these groups have excellent resources online to research the Watchtower's history. If you haven't been to these already, here are a few you can give her:
http://www.biblestudents.net/history/
http://www.ctrussell.us/
http://www.strictlygenteel.co.uk/
(This one is JW friendly ... it may be better received as a webpage okay to visit).
Good luck!