Hello!

Tsunade NPC refreshes regularly so the books in it will be cycled and rotated each time. For buying a book, it will require you to spend your Coupons/Ingots and/or Seal Scrolls. Additional information, Tsunade NPC was never released fully in our version before but accidentally went live only together with the other updates when we got a version upgrade last time. Anyways, thank you for your inquiry and have a nice day.