They didn't pursue it further because they knew that they had no chance of prevailing in court: a trademark for a soft drink doesn't extend into a totally different domain, like books. If he would have replied "go ahead and sue me, you assholes", they would have reacted exactly the same way. Just like many of today's copyright trolls, Coca Cola was trying to intimidate Grove Press into capitulating without ever intending to take the matter to court.