I'm probably being too critical, but I do wonder how I overlook things like that on each of the ten, twelve or fifteen occasions that I have read any particular passage. It's not something that's limited to my longer writings; I proof-read appallingly no matter what I'm writing (as you've probably noticed already), and it irritates the heck out of me. To a degree that is no doubt slightly obsessive, I really, really, really wish that I didn't miss mistakes in my proofs.
Hopefully I've done enough to ensure that the book is professionally presented. Recently I've read a number of books which at least give me hope that th eodd mistake may not sink the ship. Perhaps because I've become sensitized to it, I've been noticing lots of typographical errors in successful works. While I would never assume to compare myself favourably with established and successful authors, at least I know that I'm not alone in having the occasional minor error creep into the finished work...
Now to sort out the marketing of this thing!