Cofty,
To get back on this thread's focus as you requested, I would say "Why not?". After all, haven't religious people, notably amongst them, the Jehovah's Witnesses, been attempting to proselityze others since our inception? What moral authority would I have if I would criticize an atheist for attempting to proselytize a believer? My criticism goes to those who criticize the Witnesses for preaching religion all the while themselves are caught doing exactly the same thing for science (in this case, atheism wrapped in science) on every possible ocasion. That's all.
Is there merit in the endeavour? If the goal is to cause a believer to lose faith in the divine being (in abstract, whomever that person considers to be his/hers divinity, Jehovah or someone else), I think that in itself it has no merit whatsoever. If, on the other hand, the goal is to help someone to use reason to reach a more balanced view of life and the cosmos, and better separate what is 'material' from what is 'spiritual', then I concede that there might be some merit to it.
However, I would add, given the title of the book, its purpose is very clear.
Eden