There is, practically no chance for a casual player to even see those rare books, so why not to allow them to buy and enjoy? This game should be open and friendly for players with real life duties, families etc. What happens when a casual player with 100 hours in game and a full set of books on board of his shiny ship, meets in open world a veteran/fanatic with thousand hours of experience? Poor guy will be sunk, with books or without.