Part of the Twenty Books in Twenty Days series.
I was an atheist long before the four horsemen arrived on the scene in the mid-2000s. Their writing did clarify my thinking and made me consider aspects that I’d not come across before. Like How to be a Liberal, it also gave me names for concepts that I had previously been aware of.
I probably could have put any of the Dawkins, Harris, Hitchens or Dennett [affiliate links] books in the list, but there’s something about Hitchens takes-no-prisoners writing that pipped the others to the post here. (I found Dennett’s a little disappointing, to be honest. Probably a good thing I never did philosophy at university.)