If the book is hard to come by and you are happy to keep it as is (despite the less-than-honest listing), then keep the book, and leave the feedback as is. You've been honest and fair in your feedback, only marking the seller low in the areas you were not happy about.
If you are unhappy with the item as it is and want the refund, by all means take him up on his offer. IMO, you should probably still leave the feedback as is, though. Again, you're being perfectly honest about the situation. Personally, if I am a seller, I would be inclined to be over-descriptive when it comes to 'faults' in what I'm selling, so this kind of thing is less likely to happen. The seller here didn't appear to take that type of care or was possibly hoping that you would give him positive feedback anyway, or perhaps he could buy his way out of it.